[mypyc] optimise startswith and endswith

[r3m0t]

Description

Relates to mypyc/mypyc#644

Test Plan

Includes unit test

[TH3CHARLie]

LGTM and thanks for adding more primitives to mypyc!

@JukkaL Do you think is it worth to implement these two ops directly in the irbuild?(I've checked and it's possible to do so)

[JukkaL]

@TH3CHARLie Except for the most performance critical primitives it's better to use a wrapper function, since they are easier to write and maintain. If these turn out to be bottlenecks in some (realistic) benchmarks, we can consider implementing these in irbuild.

[JukkaL]

I checked that this improves the str_methods microbenchmark (in https://github.com/mypyc/mypyc-benchmarks) from 1.3x faster than CPython to 1.9x faster (on Linux)!

The results should automatically update overnight here so we can see how it performs on the benchmarks server: https://github.com/mypyc/mypyc-benchmark-results/blob/master/reports/summary-microbenchmarks.md