0.4.0

This release has two main changes:
Firstly syn was updated to 2.0, and secondly, usize and isize fields now work correctly when cross-compiling.
However, the bit sizes must now be explicitly specified for fields with these types.

Previously there was a problem when the target architecture that used a different byte size for these types (e.g., when compiling for 32-bit i686 on an x86_64 host). The proc-macro incorrectly used the size of the host architecture (which executed the macro) instead of the target's type sizes.