emacs: Implement an incremental JSON parser

emacs: Implement an incremental JSON parser

This parser is designed to read streaming JSON whose structure is
known to the caller.  Like a typical JSON parsing interface, it
provides a function to read a complete JSON value from the input.
However, it extends this with an additional function that
requires the next value in the input to be a compound value and
descends into it, allowing its elements to be read one at a time
or further descended into.  Both functions can return 'retry to
indicate that not enough input is available.

The parser supports efficient partial parsing, so there's no need to
frame the input for correctness or performance.

The bulk of the parsing is still done by Emacs' json.el, so any
improvements or optimizations to that will benefit the incremental
parser as well.

Currently only descending into JSON lists is supported because that's
all we need, but support for descending into JSON objects can be added
in the future.