This adds basic in-memory sorting utility functions, similar to SQLite's
src/vdbesort.c. We need to improve this later with external sorting so
to support large data sets.
This also adds sorting functionality to the VDBE. Note that none of this
is wired to SQL translation yet so it's unused for now.
We need to be able to allocate a new page and insert it into a page
cache without contents for asynchronoous I/O. Let's do that by making
`contents` optional in Page. (We perhaps ought to rename it to
`inner`...)
We want to submit I/O asynchronously with io_uring, which means we can
have multiple concurrent lookups to the page that is under I/O.
Therefore, let's add three page flags: up-to-date, locked, and error, so
that the higher level layers can just look up a page, but wait if
there's I/O happening.
