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/error.rs

@@ -0,0 +1,38 @@
+// 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/>.
+
+/// The error type used by the allocators.
+#[derive(sp_core::RuntimeDebug)]
+#[cfg_attr(feature = "std", derive(derive_more::Display))]
+pub enum Error {
+	/// Someone tried to allocate more memory than the allowed maximum per allocation.
+	#[cfg_attr(feature = "std", display(fmt="Requested allocation size is too large"))]
+	RequestedAllocationTooLarge,
+	/// Allocator run out of space.
+	#[cfg_attr(feature = "std", display(fmt="Allocator ran out of space"))]
+	AllocatorOutOfSpace,
+	/// Some other error occurred.
+	Other(&'static str)
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for Error {
+	fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+		match self {
+			_ => None,
+		}
+	}
+}