FIXNUM_BITS -> ECL_FIXNUM_BITS
Assumes that ECL_FIXNUM_BITS was what was wanted (presumably prefixed with ECL_ to make it a public C API). The code used a mix of ECL_FIXNUM_BITS and FIXNUM_BITS, with only FIXNUM_BITS being defined, meaning that there also previously were bugs in main.d and ffi.d.
Addresses issue #147 (closed).