read-string constructs sets; format %x lowercase; extend/extends? on nil

simon/jolt

read-string/read now return real sets for #{...} literals (top-level and
nested) instead of the reader's {:jolt/type :jolt/set} form — the data
seams convert set forms to sets (recursing, preserving metadata and source
map-key order); clojure.edn already did this. The compiler keeps reading
via the raw reader, so set literals in code stay forms the analyzer lowers.

format %x now emits lowercase hex (Chez number->string is uppercase); %X
unchanged.

extend and extends? handle a nil target type (host tag "nil"), matching
extend-type — protocols can be extended to nil via the function form, not
just the macro.

Found porting transit/data.json and shaking out aero.

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