Change hash implementation

[emmatown]

What:
Use a different implementation of murmurhash2 https://github.com/garycourt/murmurhash-js/blob/master/murmurhash2_gc.js (react-native-web also uses this)

Why:
Closes #536
It's generally faster and smaller than that the one we use currently but it's the same algorithm so none of the hashes will change.

Chrome results

Firefox results

Safari Results

How:

Checklist:

• Documentation N/A
• Tests N/A
• Code complete

[codecov]

Codecov Report

Merging #544 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
packages/emotion-utils/src/hash.js	100% <100%> (ø)	⬆️

[greggb]

Perf results on iOS are fairly poor. Any worry about this being a bottleneck?

[emmatown]

@greggb I didn't test these on iOS but I'm guessing you're referring to Safari? While the hashmumur2 functions are slower than string-hash in Safari, they're still faster than all the hash functions are in Chrome so I don't think it'll be a bottleneck and in general it's more likely that userland code will be a bottleneck rather than a hash function.

[greggb]

@mitchellhamilton That's what I had figured (user code as bottleneck), but I thought it was worth mentioning as older phones may suffer more.

Unitless graph on an X for posterity (couldn't scroll to get numbers)