[API BREAK] expose ability to create contexts without verify or signing caps

There are a lot of cases in rust-bitcoin where we need a `Secp256k1`
which doesn't need any signing or verification capabilities, only
checking the validity of various objects. We can get away with a bare
context (i.e. no precomputation) which can be cheaply created on demand,
avoiding the need to pass around references to Secp256k1 objects everywhere.

API break because the following functions can now fail (given an insufficiently
capable context) and therefore now return a Result:

    Secp256k1::generate_keypair
    Secp256k1::sign
    Secp256k1::sign_compact