char: µoptimise UTF-16 surrogates decoding

[mina86]

According to Godbolt¹, on x86_64 using binary and produces slightly
better code than using subtraction. Readability of both is pretty
much equivalent so might just as well use the shorter option.

¹ https://rust.godbolt.org/z/9jM3ejbMx

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 28162ad has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 28162ad with merge 5e8bab9...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 5e8bab9 to master...

[rust-timer]

Finished benchmarking commit (5e8bab9): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0