emacs: Streaming S-expression parser

This provides the same interface as the streaming JSON parser, but
reads S-expressions incrementally.  The only difference is that the
`notmuch-sexp-parse-partial-list' helper does not handle interleaved
error messages (since we now have the ability to separate these out at
the invocation level), so it no longer takes an error function and
does not need to do the horrible resynchronization that the JSON
parser had to.

Some implementation improvements have been made over the JSON parser.
This uses a vector instead of a list for the parser data structure,
since this allows faster access to elements (and modern versions of
Emacs handle storage of small vectors efficiently).  Private functions
follow the "prefix--name" convention.  And the implementation is much
simpler overall because S-expressions are much easier to parse.

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