Code simplification; no idea why the Context structs were being
heap-allocated piecemeal. There aren't many and they're small,
and made even smaller with this commit.
This also gets rid of the Context.cache backreference by just
accompanying the Context* with MMapCache* to the few private
Context* operations, resolving the context ID to a Context* at
the single public entrypoint: mmap_cache_fd_get(), which asserts
on the value being within bounds. The Context.id member and
checks are also removed, as there's far less opportunity for
these things to become inconsistent now.
Of course this drops all context_{add,free}() related code,
making for a tasty diffstat.
Vito Caputo
7580b0d869