Refactor: use objects to represent compilers and archiver

[awvwgk]

This patch doesn't change any functionality, but just shuffles things around.

• Fortran and C compiler are represented by thin compiler_t object
• archiver is represented by thin archiver_t object
• allows easier abstraction by moving from type(archiver_t) to class(archiver_t), allocatable later

Relevant for #442, #449

[LKedward]

Thanks, this is a useful refactoring. Feel free to incorporate #449 directly into this PR instead if easier.

[awvwgk]

This branch has diverged from the current main line, I'm going to close it, since it is easier to reimplement the changes again than updating the branch.

This branch can be resurrected from 8325a09 if needed.