Avoid the merge derive macro in rustbuild

Avoid the merge derive macro in rustbuild

The task of the macro is simple enough that a decl macro is almost ten
times shorter than the original proc macro. The proc macro is 159 lines
while the decl macro is just 18 lines.

This reduces the amount of dependencies of rustbuild from 45 to 37. It
also slight reduces compilation time from 47s to 44s for debug builds.