Rust/Marvin: add slice-only functions

[jdraaijer-microsoft]

Instead of re-using the offset/length arguments, we can simply construct a slice and operate on that, making the code a lot more concise and about 33% faster.

Also increased test coverage to test non-default offsets and lengths for correct output.

The first commit adds only the slice methods & equality tests, the second one modifies the tests more extensively.

Benchmark

[jdraaijer-microsoft]

Closing in favor of #73