Janet 1.8.0

Version 1.8.0! Mostly small fixes and a number of additions to the core library. Also exposes signals to Janet programs for custom control flow, which previously was only available from C.

• Add reduce2, accumulate, and accumulate2.
• Add lockfiles to jpm via jpm make-lockfile and jpm load-lockfile.
• Add os/realpath (Not supported on windows).
• Add os/chmod.
• Add chr macro.
• Allow _ in the match macro to match anything without creating a binding
  or doing unification. Also change behavior of matching nil.
• Add :range-to and :down-to verbs in the loop macro.
• Fix and and or macros returning nil instead of false in some cases.
• Allow matching successfully against nil values in the match macro.
• Improve janet_formatc and janet_panicf formatters to be more like string/format.
  This makes it easier to make nice error messages from C.
• Add signal
• Add fiber/can-resume?
• Allow fiber functions to accept arguments that are passed in via resume.
• Make flychecking slightly less strict but more useful
• Correct arity for next
• Correct arity for marshal
• Add flush and eflush
• Add prompt and return on top of signal for user friendly delimited continuations.
• Fix bug in buffer/blit when using the offset-src argument.
• Fix segfault with malformed pegs.