Correct function prototypes and avoid unused parameter warnings.
Remove a VERIFY_CHECK for >=0ness on an unsigned type.
Avoid a shadowed variable.
Fix varrious signed/unsigned comparisons.
Avoid unsigned comparison in scalar arith.
Signed/unsigned comparisons in tests.
Avoid forward static decl of undefined functions, also fix a paren warning in the tests.
field_gmp's negate doesn't need to use the magnitude argument.
Enable warnings.

Wno-unused-function is used for the moment because of the checking
 functions which are currently only used by VERIFY but are not (yet?)
 ifdefed out in normal builds.