fp_converter: Properly separate out symbolic glue code. (#5501)

fp_converter: Properly separate out symbolic glue code. (#5501)

File fp_converter.h encapsulates the symbolic glue code for symFPU and
implements the actual word-blaster (class FpConverter).
This header should not be included in any other headers in order to no
pull in symFPU headers where it's not needed. However, it was included
in src/theory/fp/theory_fp.h.

This properly separates it out and does some clean up in TheoryFP, which
still needs more love to conform to code style guidelines, and also
more documentation. More love and documentation is postponed to future
PRs to make reviewing easier.