bigint: constant-time operations on arbitrarily large integers

Bigint provides constant time operations on big integers. This module is limited
in scope, therefore the user must exercise caution and read the documentation
carefully to avoid misuse. Restrictions apply to the compatibility of
differently-sized big integers, and some functions require an uneven modulo.

A big integer is an array of [[word]] and must be encoded using [[encode]],
[[encodemod]] or [[encodereduce]]. See [[encodelen]] on how to calculate the
required size of the array. The big integer will also store its announced bit
length, i.e. the number of bits that are actually used to store its value; and
the effective word length, i.e. the number of words that are actually used to
store the value. The value may be decoded back to its byte format by [[decode]].

Repeated modular multiplication is supported via montgomery multiplication. See
[[tomonty]] and [[frommonty]] on how to convert from and back to this format and
[[montymul]] for the actual multiplication operation.

This is a low-level module which implements cryptographic primitives. Direct
use of cryptographic primitives is not recommended for non-experts, as
incorrect use of these primitives can easily lead to the introduction of
security vulnerabilities. Non-experts are advised to use the high-level
operations available in the top-level [[crypto::]] module.

Be advised that Hare's cryptography implementations have not been audited.
