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

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.

host/chez/compile-eval.ss

@@ -15,7 +15,10 @@
 ;; inference when the unit opted into direct-linking (jolt build --opt). Off that
 ;; path it is a pure const-fold. Loaded from the compiler image (jolt.passes).
 (define jolt-ce-run-passes (var-deref "jolt.passes" "run-passes"))
-(define jolt-ce-read (var-deref "clojure.core" "read-string"))
+;; The compiler reads source as FORMS (set literals stay {:jolt/type :jolt/set},
+;; which the analyzer lowers) — the raw reader, not clojure.core/read-string,
+;; whose data conversion would turn those into real sets.
+(define jolt-ce-read jolt-read-form-raw)
 
 ;; The spine ALWAYS runs with the full clojure.core prelude loaded, so a clojure.*
 ;; ref must lower to var-deref (resolved from the prelude), not trip the emitter's