The `io::Write` trait is somewhat worthless, being implemented for nothing, yet
`Read` remains fully functional. This also allows using its polyfills _without_
requiring `alloc`.
Opportunity taken to make `schnorr-signatures` not require `alloc`.
This will require a version bump before being published due to newly requiring
the `alloc` feature be specified to maintain pre-existing behavior.
Enables resolving https://github.com/monero-oxide/monero-oxide/issues/48.