Removing some workarounds for big integers

[Philipp]

Hi,

now that Emacs has big integers, would it make sense to remove some of
the workarounds for lack of big integer support that are still present?
Specifically, the ‘make_fixnum_or_float’ and ‘INTEGER_TO_CONS’ macros.
A replacement that would generate a fixnum or bignum depending on the
input value seems strictly superior.  (It would require a bit more
memory, but I don’t think memory pressure is an issue here.)

Philipp