Optimized the BigInteger parser function used when PORTABLE is defined; now, it doesn't create any intermediate strings via slicing.
Changed the BigRationalLarge type to a normal class instead of a single-case discriminated union because the F# compiler doesn't (yet) optimize single-case DU types as well as it could.
Removed the largely-unused BigRationalLargeImpl and BigRationalLarge modules; functions that were actually used were moved into members of the BigRationalLarge type.
Merge pull request #199 from jack-pappas/fs-bignum-optimized

BigNum Optimizations