Jolt's dynamic-var binding stack was a single global array, so concurrent go
blocks interleaved each other's bindings and a go block didn't see the bindings
in effect when it was spawned.
Move the binding stack into Janet's fiber-local dyn (:jolt/binding-stack): each
fiber (go block) lazily gets its own array, so bindings can't interleave. Janet
ev/go fibers inherit the parent's dyn, but go-spawn now snapshots the binding
stack at spawn time and installs a private copy in the new fiber — Clojure
binding conveyance. A go block's own (binding ...) shadows the conveyed frame.
types.janet: cur-binding-stack / snapshot-bindings / install-bindings; var-get/
var-set/push/pop use the fiber-local stack. async.janet: go-spawn conveys.
spec: core.async/binding-conveyance (4 cases — conveyance, isolation, no leak to
root, inner shadowing). jpm test green.
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