util: make STATIC_ASSERT usable at file scope #1952

Replace the switch-based STATIC_ASSERT, which is a statement and thus
only usable inside functions, with a declaration-like macro. It uses
_Static_assert in C11 mode and, as an extension, with GCC >= 4.6 and
Clang in pre-C11 modes. Other compilers fall back to declaring an
extern array whose size is negative if the assertion fails.

Since STATIC_ASSERT now expands to a declaration, move its use in
secp256k1_ge_impl_to_bytes before VERIFY_CHECK to avoid mixing
declarations and code, which C89 forbids.

Now that STATIC_ASSERT can be used at file scope, the
secp256k1_assumption_checker function, which existed only to provide a
function context for the assertions, is no longer needed.