Expose std_shims::io on core

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.

crypto/embedwards25519/src/lib.rs

@@ -2,7 +2,6 @@
 #![doc = include_str!("../README.md")]
 #![cfg_attr(not(feature = "std"), no_std)]
 
-#[cfg(feature = "alloc")]
 #[allow(unused_imports)]
 use std_shims::prelude::*;