optimize Eq implementation for paths

[the8472]

Filesystems generally have a tree-ish structure which means paths are more likely to share a prefix than a suffix. Absolute paths are especially prone to share long prefixes.

quick benchmark consisting of a search through through a vec containing the absolute paths of all (1850) files in compiler/:

# old
test path::tests::bench_path_cmp                                  ... bench:     227,407 ns/iter (+/- 2,162)

# new
test path::tests::bench_path_cmp                                  ... bench:      64,976 ns/iter (+/- 1,142)

[rust-highfive]

r? @kennytm

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[the8472]

@rustbot label T-libs-impl

[kennytm]

@bors r+ rollup=never

[bors]

📌 Commit 53d71c1 has been approved by kennytm

[bors]

⌛ Testing commit 53d71c1 with merge 0041ef71d6a83e496a616867e6fbe52097b67a2c...

[rust-log-analyzer]

The job dist-mipsel-linux failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

💔 Test failed - checks-actions

[the8472]

failed to get 200 response from https://crates.io/api/v1/crates/byteorder/1.4.3/download, got 502

looks like a spurious error.

@bors retry

[bors]

⌛ Testing commit 53d71c1 with merge 9fef8d9...

[bors]

☀️ Test successful - checks-actions
Approved by: kennytm
Pushing 9fef8d9 to master...