Bring net/parser.rs up to modern up to date with modern rust patterns

[Lucretiel]

The current implementation of IP address parsing is very unidiomatic; it's full of if / return / is_some / is_none instead of ?, loop with manual index tracking; etc. Went through and did and cleanup to try to bring it in line with modern sensibilities.

The obvious concern with making changes like this is "make sure you understand why it's written that way before changing it". Looking through the commit history for this file, there are several much smaller commits that make similar changes (For instance, 3024c14, 4f3ab49, 79f8764), and there don't seem to be any commits in the history that indicate that this lack of idiomaticity is related to specific performance needs (ie, there aren't any commits that replace a for loop with a loop and a manual index count). In fact, the basic shape of the file is essentially unchanged from its initial commit back in 2015.

Made the following changes throughout the IP address parser:

• Replaced all uses of is_some() / is_none() with ?.
• "Upgraded" loops wherever possible; ie, replace while with for, etc.
  • Removed all cases of manual index tracking / incrementing.
• Renamed several single-character variables with more expressive names.
• Replaced several manual control flow segments with equivalent adapters (such as Option::filter).
• Removed read_seq_3; replaced with simple sequences of ?.
• Parser now reslices its state when consuming, rather than carrying a separate state and index variable.
• read_digit now uses char::to_digit.
• Added comments throughout, especially in the complex IPv6 parsing logic.
• Added comprehensive local unit tests for the parser to validate these changes.

[rust-highfive]

r? @cramertj

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[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

##[section]Starting: Linux mingw-check
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Current agent version: '2.168.2'
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Environment: ubuntu-16.04
Version: 20200512.2
Included Software: https://github.com/actions/virtual-environments/blob/ubuntu16/20200512.2/images/linux/Ubuntu1604-README.md
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Agent running as: 'vsts'
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Downloading task: Bash (3.163.2)
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##[section]Starting: Checkout rust-lang/rust@refs/pull/72369/merge to s
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Version      : 1.0.0
Author       : Microsoft
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Step 7/7 : ENV SCRIPT python3 ../x.py test src/tools/expand-yaml-anchors &&            python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu &&            python3 ../x.py build --stage 0 src/tools/build-manifest &&            python3 ../x.py test --stage 0 src/tools/compiletest &&            python3 ../x.py test src/tools/tidy &&            python3 ../x.py doc --stage 0 src/libstd &&            /scripts/validate-toolstate.sh
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Successfully built 537a01811900
Successfully tagged rust-ci:latest
Built container sha256:537a018119009dc218456238dec90b5530050db1e2a1e166550c218003f6159d
---
    Checking panic_unwind v0.0.0 (/checkout/src/libpanic_unwind)
error[E0425]: cannot find value `p` in this scope
   --> src/libstd/net/parser.rs:116:21
    |
116 |             *slot = p.read_separator('.', i, |p| p.read_number(10, 3, 0x100))? as u8;

error: aborting due to previous error

For more information about this error, try `rustc --explain E0425`.
---
  local time: Wed May 20 04:03:32 UTC 2020
  network time: Wed, 20 May 2020 04:03:32 GMT
== end clock drift check ==

##[error]Bash exited with code '1'.
##[section]Finishing: Run build
##[section]Starting: Checkout rust-lang/rust@refs/pull/72369/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
Author       : Microsoft
Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
==============================================================================
Cleaning any cached credential from repository: rust-lang/rust (GitHub)
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[Dylan-DPC-zz]

r? @dtolnay maybe?

[dtolnay]

Thanks, this looks great. I didn't review too closely but it passes this fuzz test so I have reasonable confidence that the behavior matches the original.
https://gist.githubusercontent.com/rust-play/8d17b0e3789782ac09d7387c41943d65/raw/dc9728b304d72ba64553b1ea361559ef9798dd94/playground.rs

[dtolnay]

@bors r+

[bors]

📌 Commit e02b3fb00bfe8baf185017db4cff1939a06814d5 has been approved by dtolnay

[bors]

⌛ Testing commit e02b3fb00bfe8baf185017db4cff1939a06814d5 with merge 6b9a1b4007948c3e6b72eb81b83a04913dede9c7...

[bors]

💥 Test timed out

[dtolnay]

@bors r+

[bors]

📌 Commit 3ab7ae3 has been approved by dtolnay