bitreq: Test `std` and `tokio-rustls` in CI

[tcharding]

cargo rbmt provides a way to test exact feature combinations. After #563 we should test std and tokio-rustls and probably a bunch of other sane cobinations.

As an example this is from bitcoin-primitives:

[package.metadata.rbmt.test]
exact_features = [
    ["alloc", "hex"],
    ["alloc", "serde"],
    ["hex", "serde"],
    ["std", "hex"],
    ["std", "serde"],
    ["alloc", "hex", "serde"],
    ["std", "hex", "serde"],
]