feat(kdl): kdl_match function

[tychoish]

This is definitely a WIP (though I think it's relatively safe, as
these things go,) and I mostly wanted to see what it would take to do
something like this.

KDL is a pretty neat way of expressing "documents" and comes with a
pretty complete ecosystem of tools and the query language means this
is pretty easy to support without all that much heavy lifting.

I wanted to stop and get feedback before digging further in, and
obviously see what CI had to say for itself.

In my mind there were a couple of other interesting
things/possibilites that I wanted to mention, because I had been
thinking about it.

• I really wanted to have a way of type alaising a field so that it
  could have KDL data in it and GlareDB (and in particular the storage
  engine) could think about optimizations within that. Adding UDFs
  seems tractable, and while I think ultimately we'd want to store KDL
  data in UTF-8 fields: it'd be nice to be able to error if someone
  passed the wrong kind of field.

• It's a project for another day, but it would be interesting to see
  if you could make a binary format based on the data model:

  • length prefixes for more efficent skipping.

  • store the encoding format for numbers (int32 for number of 32bit
    integers of storage; plus 32bits for a bitmask to store
    [is_float;is_varint,is_string])

  • maybe use the sign bit of the length prefixes for "streaming" to
    indicate if this length includes the end of
    document/message/component or if after the provided length the
    next length is a continuation of the same object.