Add a feature matrix section to the `bitcoin` CI script as we do in
`hashes`. This means:
- test with no default features
- test with all individual features
- test with all combinations of two features
Note, with this applied all features and optional dependencies are
included in `FEATURES` (excluding `secp-lowmemory`).
The feature matrix test gets run for stable and MSRV toolchains.
b72f1d70e5