The version 1.63 satisfies our requirements for MSRV and provides
significant benefits so this commit bumps it. This commit also starts
using some advantages of the new MSRV, namely namespaced features, weak
dependencies and the ability to use trait bounds in `const` context.
This however does not yet migrade the `rand-std` feature because that
requires a release of `secp256k1` with the same kind of change - bumping
MSRV to 1.63 and removing `rand-std` in favor of weak dependency.
jike
Martin Habovstiak
Jose Storopoli
Tobin C. Harding