Make Pieces immutable and return them from cache

[akx]

This PR makes Piece a frozen dataclass and adds a cache for them. For end users (and the library itself of course) this change is invisible API-wise.

However, there is a visible performance increase (even if the test suite is not necessarily the best indicator):

hyperfine 'git checkout master; git clean -fdx; py.test --count=10 test.py' 'git checkout immutable-pieces; git clean -fdx; py.test --count=10 test.py'

Benchmark 1: git checkout master; git clean -fdx; py.test --count=10 test.py
  Time (mean ± σ):      6.405 s ±  0.411 s    [User: 5.249 s, System: 0.404 s]
  Range (min … max):    6.120 s …  7.537 s    10 runs

Benchmark 2: git checkout immutable-pieces; git clean -fdx; py.test --count=10 test.py
  Time (mean ± σ):      6.246 s ±  0.131 s    [User: 5.218 s, System: 0.394 s]
  Range (min … max):    6.126 s …  6.591 s    10 runs

Summary
  'git checkout immutable-pieces; git clean -fdx; py.test --count=10 test.py' ran
    1.03 ± 0.07 times faster than 'git checkout master; git clean -fdx; py.test --count=10 test.py'

This will also likely have a nice memory usage effect.

Let me know what you think :)

[niklasf]

Hi, thanks! I like this patch.

Even as is, pieces are intended to be immutable (no setters, provides __hash__). But that's not documented, so this is technically still a bc break and needs to wait for a future version 2.x.