What we need is an open-source alternative of Mathematica's core language.
Common Lisp has a robust and comprehensive numerics facility. Poking around the CLHS and reading about ratios, fixnums, bignums, etc., will lead you to the conclusion that you can get your numeric work done (albeit in many cases much more slowly than you would like).
Add one of the pattern matching and term rewriting packages and you'll be very close to the foundation of the core of Mathematica. It will be a SMOP and adding to the library of facts enabling recognition of reducible terms.