Support tower fields

[mhostetter]

Perhaps for large fields GF(p^m), that would normally use explicit calculation, a tower of GF(p^e^k) could be constructed. It would be such that GF(p^e) would use lookup tables. Then the degree-k extension of GF(p^e) would use explicit calculation. Certainly logarithms would be faster. Perhaps other arithmetic would be faster too.

This "tower-like" arithmetic will be necessary for Galois rings.

Perhaps add a parameter like allow_tower=True to galois.GF().