libtfm

This is a trivial port of the Tom's Fast Math (TFM) bignum library to the Cryptech environment. We use a git submodule to pull the package from GitHub, we verify that the SHA-256 digest of what we got from GitHub matches the version we tested, then we build the library with the options we want.

See tomsfastmath/doc/tfm.pdf for API details.

In theory, the need for most (perhaps all) of this will go away when more of the bignum math is implemented in Verilog. Part of the reason for using the TFM library is that its extremely modular structure make it easy for us to link in only the functions we need.