Replace the correctness-only transient aliases with real mutable scratch
collections via host interop:
- transient vector -> a Janet array; conj!/assoc!/pop! mutate in place
- transient map -> a Janet table keyed by canonical key (collection keys still
compare by value); assoc!/dissoc!/conj! mutate in place
- transient set -> a Janet table; conj!/disj! mutate in place
- persistent! freezes back to a pvec / phm / phs
- count/nth/get/contains? work on transients; transient? predicate added
Building a map/set this way avoids the persistent path's per-step bucket-array
copying (transient map build ~35% faster at 20k here); vectors are comparable
since pvec conj is already ~O(1). The mutating ops return the transient and the
source collection is untouched.
spec/transients-spec (34 cases). conformance 218/218, jpm test green.
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