adjust runtime page size APIs

* fix merge conflicts
* rename the declarations
* reword documentation
* extract FixedBufferAllocator to separate file
* take advantage of locals
* remove the assertion about max alignment in Allocator API, leaving it
  Allocator implementation defined
* fix non-inline function call in start logic

The GeneralPurposeAllocator implementation is totally broken because it
uses global state but I didn't address that in this commit.