SecureRandom has been dropped in favor to Random::Secure (renamed
from Random::System) which is now the secure source for random
numbers suitable for cryptography.

Moves SecureRandom#random_bytes(Bytes) into Random::Secure, and
implements a generic Random#random_bytes(Bytes) to fill a Bytes with
random numbers from any PRNG.

Moves SecureRandom methods such as #base64 and #uuid into Random, so
any PRNG implementation may use these methods; they should only be
used with a secure PRNG thought (except maybe in test suites). This
change allows to swap PRNG at will. For example swap Random::Secure
for Random::ISAAC (seeded from Random::Secure), or use a
cryptographically secure but slow PRNG in production, and an
insecure but fast and determinable PRNG in test suites, for example.

…-random

Merge SecureRandom into Random and Random::Secure

- Renames Random::System to Random::Secure.
- Removes SecureRandom.
- Moves #uuid, #hex and #base64 methods to Random.
- Moves SecureRandom#random_bytes(slice) to Random::Secure.
- Implements generic Random#random_bytes(slice).