This introduces new cryptozoology.utils.encoding and

cryptozoology.utils.random modules.
encoding provides utilities for encoding to/decoding from URL-safe

Base64 with padding removed and type checking agdded. It also provides

encoding for length-prefixed strings, which is useful in AAD and HKDF

information.
random provides types and generation for nonces and salts of arbitrary

or common sizes.

Unit tests pass.
Made use of these in other changes.