Move "wasm" allocator into its own crate (#4716)

This moves the wasm-allocator (`FreeingBumpHeapAllocator`) into its own
crate `sp-allocator`. This new crate can theoretically provide multiple
different allocators. Besides moving the allocator, this pr also makes
`FreeingBumpHeapAllocator` compile on `no_std`.

substrate/primitives/runtime/src/traits.rs

@@ -458,36 +458,18 @@ impl<H: PartialEq + Eq + Debug> CheckEqual for super::generic::DigestItem<H> whe
 	}
 }
 
-macro_rules! impl_maybe_marker {
-	( $( $(#[$doc:meta])+ $trait_name:ident: $($trait_bound:path),+ );+ ) => {
-		$(
-			$(#[$doc])+
-			#[cfg(feature = "std")]
-			pub trait $trait_name: $($trait_bound +)+ {}
-			#[cfg(feature = "std")]
-			impl<T: $($trait_bound +)+> $trait_name for T {}
-
-			$(#[$doc])+
-			#[cfg(not(feature = "std"))]
-			pub trait $trait_name {}
-			#[cfg(not(feature = "std"))]
-			impl<T> $trait_name for T {}
-		)+
-	}
-}
-
-impl_maybe_marker!(
+sp_core::impl_maybe_marker!(
 	/// A type that implements Display when in std environment.
-	MaybeDisplay: Display;
+	trait MaybeDisplay: Display;
 
 	/// A type that implements Hash when in std environment.
-	MaybeHash: sp_std::hash::Hash;
+	trait MaybeHash: sp_std::hash::Hash;
 
 	/// A type that implements Serialize when in std environment.
-	MaybeSerialize: Serialize;
+	trait MaybeSerialize: Serialize;
 
 	/// A type that implements Serialize, DeserializeOwned and Debug when in std environment.
-	MaybeSerializeDeserialize: DeserializeOwned, Serialize
+	trait MaybeSerializeDeserialize: DeserializeOwned, Serialize;
 );
 
 /// A type that provides a randomness beacon.