~32% faster

~3% faster

Montgomery<_> only works for odd n, so attempting to construct an instance
for an even number results in a panic!

The most obvious solution is to special-case even numbers.

This is more descriptive, as semiprime are the products of 2 primes;
all semiprimes are composite, but not all composite numbers are semiprime.

test() takes a modulus that is known to not be even or <2 (otherwise the
Montgomery value could not be constructed), so those checks can be hoisted
into is_prime() and out of the critical path.

Detected by the testsuite improvement just prior.

- `DoubleInt::Double` renamed to `DoubleWidth`
- `{as,from}_double()` renamed to `{as,from}_double_width()`.

This should hopefully clarify that this is not a “double precision”
floating-point type, but an integer type with a larger range (used
for storing intermediate results, typ. from a multiplication)

It was unused, the debug assertions only need `to_u128`.