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.

common/std-shims/src/collections.rs

@@ -1,7 +1,7 @@
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+pub use extern_alloc::collections::*;
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+pub use hashbrown::{HashSet, HashMap};
+
 #[cfg(feature = "std")]
 pub use std::collections::*;
-
-#[cfg(not(feature = "std"))]
-pub use alloc::collections::*;
-#[cfg(not(feature = "std"))]
-pub use hashbrown::{HashSet, HashMap};