Fixed bigfloat functions that assumed (fixnum? x) means x fits in a _long
(not true on Win64)
Hopefully fixed dangling pointer errors that broke `math/bigfloat' on Win64.
It apparently had no _long/_int mismatches, but GC on Win64 will run between
creating an `_mpz' and using its value after passing it as an output argument
to MPFR functions. That doesn't seem to happen on 64-bit Linux or Mac. No
idea why, but Win64 exposed the problem so... that's good, I guess.
Rewrote `rational->bigfloat' to not use GMP's rationals
More/better bigfloat tests
Added bigfloat stress test w/ weak leak detection
Reenabled custodian shutdown callback that clears MPFR constants, because it
seems to work now
Removed `mpfr-available?' because it would only return non-#f
