Move Rng failure from `Secp256k1::new` to functions that actually use randomness

Verifying signatures does not require any randomness, but requires the user
to create a `Secp256k1` object nonetheless (this is just a way to guarantee
that `init` is called --- an alternate API would be to have an independent
unsafe `verify` function). If a Rng can't be created, rather than failing
the `Secp256k1` initialization, fail the functions that actually try to use
the Rng.

This way signing and verifying, which require no randomness beyond that input
to them, will work correctly.

To avoid checking for a working Rng on each call to `generate_keypair` and
`generate_nonce` (which is probably trivial next to the cost of actually
generating the randomness, but w/e, user knows best), the user should use
the generation functions in the `key` module, which take an Rng as input.