Revert bpe changes to bigint.js for TBB

The compiler in Firefox 24.2.0 ESR is having a hard time with these changes,
making key generation in the TorBrowserBundle infeasible.

Reverts some of cbdc06781d9cfcf2334a3984e199771272daf672.
Merge pull request #553 from arlolra/revert

Revert bpe changes to bigint.js for TBB