Checkpoint.

* Added note at outset that `typed_alloc` is not currently for direct
  implementation.

* Added "Example usage" section with one example motivated by a
  `Vec`-like library.  (Tried to add an example of a library using
  `InstanceAlloc`, but I could not make something small that also
  motivated use of `InstanceAlloc`.)

* Added sub-sections that follow `typed_alloc` module API.

* Removed `BaseAlloc` and its `tightly_gc_integrated`

* Attempted to clarify overflow behavior on `ArrayAlloc` methods.

* Removed the weak note I was worried about in `deinit_range`.  Also,
  alpha-renamed `deinit_range` to `reinit_range`.

* Added references to "fully-backed" terminology in `StdAlloc` and
  `Direct`.

* Added an unresolved question about whether I should represent the
  default allocator as just `()`.
Removed mention of Reaps from the document.
Added note about lack of  libarena integration.
added yet another note trying to push the reader towards typed_alloc.
moved AllocCore to an appendix.
removed reference to MemFragSolvedP; its not sufficiently relevant here.