fix: Use `read_u32` instead of `read_i32` to prevent wrong size parsing

[paxbun]

The current code fails to parse SoundFonts greater than 2GB, because of code like the following:

https://github.com/sinshu/rustysynth/blob/b87a7f2329c049eb22f6345fb8e6a88b8528f5f1/rustysynth/src/soundfont_sampledata.rs#L23

When the size of the chunk is greater than 2GB, which makes the MSB be set, the resulting usize contains a value like 0xFFFFFFFF..., due to the sign extension.

This PR adds BinaryReader::read_u32 to prevent this. This PR also adds the type FourCC so BinaryReader::read_four_cc does not allocate a String redundantly.

Please refer to the following for more details in the numeric cast in Rust:

• The Reference: https://doc.rust-lang.org/reference/expressions/operator-expr.html#numeric-cast
• Demo: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&code=fn+main%28%29+%7B%0A++++let+i%3A+i32+%3D+-1%3B%0A++++println%21%28%22%7B%3AX%7D%22%2C+i+as+usize%29%3B%0A%7D%0A

[sinshu]

Thanks 😄 Your PR looks good. I'll test the code this weekend.

[sinshu]

I merged the PR. It's nice that the error types don't allocate a String 😎👍