Implement stats.bookkeeping.

This accounts for:
    - All base allocations, including arena headers;
    - All chunk headers;
    - Extent nodes used for huge allocations;
    - All allocations performed via a0{m,c}alloc.
    - Allocations made for tcache;
    - Allocations made for the quarantine;
    - Allocations made for heap profiling.

Note, however, that some of the allocations above are also accounted
for in several other stats such as stats.allocated or individual arena stats.