[DO NOT MERGE]: Try out 128bit Metrohash for incr. comp. hashes.

[michaelwoerister]

This is just for performance testing on perf.rust-lang.org. It's not meant to be merged in this state.

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[michaelwoerister]

@bors try

[bors]

⌛ Trying commit ea204a4 with merge 00beaf41c70916670fa9ef58944f59a9f0bc774c...

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[Mark-Simulacrum]

http://perf.rust-lang.org/compare.html?commit_a=fd8099f0ecc2fd3980469ec45cf834ae9a941ca3&commit_b=00beaf41c70916670fa9ef58944f59a9f0bc774c&stat=instructions%3Au

[michaelwoerister]

And thanks again, @Mark-Simulacrum!

Here's the actual comparison (red/green vs red/green):
http://perf.rust-lang.org/compare.html?commit_a=d7e73e4b1abe120520d1894b89c0e25469f41e69&commit_b=00beaf41c70916670fa9ef58944f59a9f0bc774c&stat=instructions%3Au

[michaelwoerister]

Closing, see follow-up here: #41215 (comment)