Update WyIL format for RFC#0003

[DavePearce]

This is a preparatory step for RFC#0003. The goal is to update the WyIL file format to support:

1. Fixed sized signed and unsigned integers (e.g. u32 and i64)
2. Fixed size arrays (e.g. int[3])
3. Fixed index arrays (e.g. int[i32])

However, this issue does not update other parts of the compiler, such as the parser and type checker --- these will be updated separately.