Breaking Change Proposal: Replace isize with i32 or u32 as representations for enums.

[DarthB]

At the moment the enums represented in raw.rs use isize which has different sizes (4 or 8 bytes) depending on the system.

• This PR removes the usage of isize as representation for enums and replaces it with either u32 or i32, depending on whether the underlying constants have negative values.
• It adds strum as a dependency to add a from_repr method to the enums.

This is a breaking change.