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/src/lib.rs

@@ -48,7 +48,7 @@ pub use sp_core::traits::Externalities;
 pub use sp_wasm_interface;
 pub use wasm_runtime::WasmExecutionMethod;
 
-pub use sc_executor_common::{error, allocator, sandbox};
+pub use sc_executor_common::{error, sandbox};
 
 /// Call the given `function` in the given wasm `code`.
 ///