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/client/executor/common/src/lib.rs

@@ -19,6 +19,5 @@
 #![warn(missing_docs)]
 
 pub mod sandbox;
-pub mod allocator;
 pub mod error;
 pub mod wasm_runtime;