Use usize everywhere

[GWBasic]

In Rust, indexing of arrays and general sizes are typically represented by a usize.

wave_stream currently uses a u32, because the wave header uses u32.

Initially, I believed it was best to keep with the type limitations imposed by the wave format. Instead, I believe it's simpler to keep with Rust semantics for indicating size and location; and have a runtime exception if the wave file is longer than u32::MAX;