This can be used in the future to trigger garbage collection. For now,
it provides a more useful error message in case the heap is completely
filled up.
Make sure every to-be-implemented GC can use the same interface. As a
result, a 1MB chunk of RAM is allocated on Unix systems on init instead
of allocating on demand.
* Use 64-bit integers on 64-bit platforms, just like gc and gccgo:
https://golang.org/doc/go1.1#int
* Do not use a separate length type. Instead, use uintptr everywhere a
length is expected.
