Reduce scope of template generator, split it into a separate repo

[ggutoski]

It's been decided that the template generator should generate only the following:

• EC group code (g1.go, g2.go)
• field tower (e2.go, e6.go, e12.go)
• Frobenius constants (frobenius.go)

In particular, the template generator should not produce any pairing code (except for Frobenius constants). Pairing code is too dependent on the specific curve---generating this code inside the template is just a mess.

We should split the template generator into a separate repo, similar to goff. (Maybe merge the gurvy template generator into goff?) The gurvy repo should not contain any template code. Instead it should contain only the output of the template generator plus any curve-specific code such as pairing code.