* Add craft-kiwi: the kiwi binary schema format, decoded and encoded
The generic kiwi decoder and encoder from FigCraft's .fig import and export
(crates/format/src/fig/kiwi.rs at 3b9bf8c9), moved as it is: binary schema
decode and encode, the compact text form of a schema, message decode into a
tree keyed by field name, and message encode checked against the schema.
The value budget that follows a .fig file's compressed size stays with the
.fig reader; here decode_message_within takes the budget and
DEFAULT_MAX_VALUES is the floor FigCraft used.
* craft-kiwi: no scan per field in schema decode; the text form keeps the limits
Schema::decode compared each message field id with every earlier one: 20
definitions of 10 000 fields, within every cap, took a billion
comparisons. A set per definition now.
Schema::parse applies the definition and field caps of the binary form,
looks type names up in an index instead of scanning the lines per field,
accepts a struct or message without fields, and refuses two fields of one
name and a message field id that is 0 (it ends a message) or repeated.
