Implement stats.bookkeeping.

This is a first and crude attempt at issue #163, but hopefully it's on the right track.

This attempts to account for:
    - All base allocations, including arena headers;
    - All chunk headers;
    - Extent nodes used for huge allocations;
    - All allocations performed via `a0{m,c}alloc`.

This leaves missing:
    - Allocations of `tcache_t`;
    - Allocations of `quarantine_t`;
    - Allocations made for heap profiling.

Thoughts?