Also:
 - Upgrade test from pseudorandom to proptest
 - Add method `bits` which returns bitsize

Anticipates faster reduction modulo multimodular primes.

`MASTER_TREE` is populated from a rust expression inside a
`lazy_static!` declaration, meaning that it is computed from
native rust (so fast) once per thread in which it is needed.
This allows faster reduction of large integers modulo up to
`N = 512` thirty-two-bit primes.

Specifically, `modular_inverses_sequence` and `product_tree` need to
be separate from `multimod`, but depend on the `MODULI` static ref.
Solved by introducing `residue_number_system.rs` which contains
`MODULI` and associated logic. Now `multimod.rs` depends on all
three and adds logic for dealing with multimodular integers.

Specifically: `MODULAR_INVERSES_SEQUENCE`.

SHAKE makes the PRNG expansion deterministic and comparable across
implementations.

In the given context, the operands can be assumed to have compatible
capacities already.

Also: drop bit capacity tracker. For every multimod arithmetic, the
available capacity is assumed to be enough.

Irrelevant for performance.