Signature optimization

[joe-hauns]

Optimized the signature such that no intermediate strings are created when looking up a symbol. This is espcially important for numerals.
Before this PR, whenever we wanted to find a numeral's function-id we did the following:

• turn the numeral into a string
• hash that string
• look up the symbol id from _funNames
  Now we will directly hash the number, which is way faster. Similar optimizations have been performed for looking up interpreted functions. Further I managed to shrink the codebase a bit as a sideeffect.

Also while i was on it I removed the unused flag overflownConstant as that is not necessary anymore since we're using GMP.

[MichaelRawson]

Nice.