rename shamir_params_t.t to threshold
Remove duplication in the exponent table.

There are enough modulo operations everywhere that this is a moderately
useless optimisation anyway.
Rename _shamir_get_key to shamir_get_key

Semantically, shamir_get_key becomes part of the public interface. This
is useful in certain situations.
add shamir_key_x and shamir_poly_secret
add man pages
Makefile: don't install g256_tables.h