Update on Format 2 Patch Map Spec Text

I've just sent out a PR that adds spec text for the previously missing
format 2 patch map. PR is here <https://github.com/w3c/IFT/pull/159> and
you can see a preview of the new text here
<https://garretrieger.github.io/IFT/Overview.html#patch-map-format-2>. The
format 2 encoding I landed on is fairly similar to the protobuf based
encoding
<https://github.com/w3c/patch-subset-incxfer/blob/main/ift/proto/IFT.proto#L20>
I used in the prototype.

To test that the proposed encoding would work in practice I also updated
the prototype
<https://github.com/garretrieger/patch-subset-incxfer/tree/format2> to
include an implementation of serialization/deserialization of the new
format 2 encoding. Using this I was able to get the demo
<https://garretrieger.github.io/ift-demo/> working using the new encoding.
This new encoding is more compact. In some test fonts I tried I saw a
~5-14% reduction in post woff2 compression sizes of the mapping table vs.
the previous protobuf based encoding.