* FreeingBumpAllocator: Initialize the heads to `u32::max_value()`
`self.heads` can point to an element with the index `0` in the heap.
This would make the allocator fail to reuse this element.
* Simplify the `PREFIX_SIZE` handling
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`.