Thread support for REPL
Comment rewording
InputLocks are maintained in a single hash, keyed by inputs
Guard against spurious interrupt in interruptible_region
register_ownership -> with_ownership
Only the owner can touch the @interruptible flag
Pry's critical sections should be per thread
No longer touching the the input. It's a global lock for now (:all)
Merge pull request #944 from nviennot/master

Thread support for REPL