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/allocator/src/lib.rs

@@ -0,0 +1,29 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Collection of allocator implementations.
+//!
+//! This crate provides the following allocator implementations:
+//! - A freeing-bump allocator: [`FreeingBumpHeapAllocator`](freeing_bump::FreeingBumpHeapAllocator)
+
+#![cfg_attr(not(feature = "std"), no_std)]
+#![warn(missing_docs)]
+
+mod error;
+mod freeing_bump;
+
+pub use freeing_bump::FreeingBumpHeapAllocator;
+pub use error::Error;