Simplify Readable mutability.

Simplify Readable mutability.

In anticipation of the rust-bitcoin upgrade, which incorporates its
own `io::Read` implementation, we need to make our usage compatible
with dropping `std::io` and `core2::io`.

Notably, in version 0.32.2, `bitcoin::io`'s `Read` is no longer
implemented for `&mut R where R: Read + ?Sized`, which results in
errors anytime `&mut &mut Readable` is passed instead of
`&mut Readable`.

This commit fixes those instances.