Allow field type sizes up to 128 bits

[winstonallo]

The macro now supports packed bitmap structs up to 128 bits (previously 64), accepts field sizes from u1 to u128, and generates a richer API for field and raw value access.

Macro feature expansion

• Increased the maximum supported bitmap size from 64 bits to 128 bits, and allows fields of type u1 through u128.
• The macro now generates From<Bits> for T and Deref implementations for the underlying storage type.

Closes #1