Add support for compressed samples

[NoobTracker]

I've implemented sample decompression because I hate my life or something. Sorry for the code quality, I've never really used rust before. Hopefully refactoring this is easier than writing everything from scratch :P
I've also modified how sample values are converted to f32s - previously the i8s and i16s just got divided by their respective MAX constants, but that meant that they were not always within the -1 to 1 range, -128 divided by 127 is < -1. So now they get divided by the MIN constants, *-1. Oh, and it gets rid of the floating point inaccuracies. I think it makes more sense that way, but feel free to disagree.
I've tested the code by loading a module with compressed samples and rendering it using the example code, hope I didn't miss anything.

[pr2502]

Hi, thanks for the contribution!

Would it be possible to add a test for it? If I understand it correctly the compression scheme should be lossless, so in theory it should be possible to add a test that compares an original .wav file to one produced from a compressed .is file created in something like OpenMPT?

[NoobTracker]

That sounds like a good idea, considering how messy everything is ^^
And yes, the compression is lossless. I guess I'll just generate some noise and let OpenMPT compress it. The parser doesn't support impulse tracker instrument files, does it?

[pr2502]

It does! https://docs.rs/ittech/latest/ittech/parser/fn.instrument_file.html will parse an InstumentFile from &[u8], which you can just include_bytes! in the test.

[NoobTracker]

Good to know! Test has been added, it decompresses a few KB of noise, both 16 bit and 8 bit, hopefully that catches everything I could've messed up. Not sure if I wrote the test "the proper way", I just copypasta'd your pattern effect test :D
Oh, and there's not really a way to check if a sample was compressed or not without changing the API, right?

[NoobTracker]

So yeah I am very bad at using git ... I have just fixed song messages but I have no idea how to create a new PR with just those changes so I guess I'll add them to this PR?

[NoobTracker]

Continued in #3 . I'm really sorry. sigh
(in my defense, the pain in my foot caused by a medical operation really doesn't improve my concentration)