Add benchmarks for bitfields

[Robbepop]

This adds benchmarks for the bitfield implementation.

Compares the performance of getters and setters generated by the macro with the performance of a handwritten and hard coded implementation for the same spec.

Useful

• To learn how to generate macro code in a way that the compiler is able to heavily optimize it later.
• For beginners to understand how to manipulate data on a bit level.

Note

• Feel free to adjust the implementation in later commits to make it opt-in testable.

Benchmarks

Below some benchmarks comparing the hand-written implementation of this PR with the macro-generated implementation of the bitfield implementation in my fork. The performance of both seem to be similar.

test generated::get_a   ... bench:          99 ns/iter (+/- 4)
test generated::get_b   ... bench:          98 ns/iter (+/- 4)
test generated::get_c   ... bench:          98 ns/iter (+/- 14)
test generated::get_d   ... bench:          97 ns/iter (+/- 2)
test generated::get_e   ... bench:          99 ns/iter (+/- 2)
test generated::set_a   ... bench:         481 ns/iter (+/- 19)
test generated::set_b   ... bench:         627 ns/iter (+/- 37)
test generated::set_c   ... bench:         507 ns/iter (+/- 35)
test generated::set_d   ... bench:         622 ns/iter (+/- 24)
test generated::set_e   ... bench:         459 ns/iter (+/- 16)
test handwritten::get_a ... bench:          99 ns/iter (+/- 3)
test handwritten::get_b ... bench:         102 ns/iter (+/- 14)
test handwritten::get_c ... bench:         102 ns/iter (+/- 8)
test handwritten::get_d ... bench:         100 ns/iter (+/- 20)
test handwritten::get_e ... bench:          98 ns/iter (+/- 6)
test handwritten::set_a ... bench:         582 ns/iter (+/- 20)
test handwritten::set_b ... bench:         614 ns/iter (+/- 35)
test handwritten::set_c ... bench:         533 ns/iter (+/- 18)
test handwritten::set_d ... bench:         606 ns/iter (+/- 21)
test handwritten::set_e ... bench:         456 ns/iter (+/- 21)

[Robbepop]

Just added some tests to the benchmarks to assert that their implementations are actually correct and fixed some bugs while doing so.

You can invoke the benchmark tests with: cargo test --benches