We'll use this allocator in a follow-up CL to:
- allocate speculative sizes of memory for a module that's being
compiled (e.g. 2*size of wasm code).
- each module will own such a sub-pool, and then use it to allocate
contiguous chunks of memory for code.

The underlying assumptions for the chosen allocation strategy is that:
- the allocation granularity for pools is 1 page, so that no one page
is owned by more than one wasm module
- typical pool sizes (given module sizes) are multiple pages.
- modules and module instances are typically few and long lived. Typically,
we expect one module and one instance. 

This means we shouldn't expect fragmentations that lead to code being
non-allocatable, or prohibitively many ranges.

The data structure just manages ranges of addresses. Virtual memory management
will be separate, as part of the responsibility of a "WasmHeap"
that will be introduced in the future. So will concurrency control.

Bug: 
Change-Id: Id99f46d10c25553b013054d994760f3c2a737c39
Reviewed-on: https://chromium-review.googlesource.com/669296
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48053}