Janet 1.2.0

This release brings a number of improvements to the language, including shorthand anonymous function literals, and new functions in the core library. You can also use jpm to create binary executables from Janet code (provided you have a C compiler). Also comes with a handful of bug fixes.

• Add take and drop functions that are easier to use compared to the
  existing slice functions.
• Add optional default value to get.
• Add function literal short-hand via | reader macro, which maps to the
  short-fn macro.
• Add int? and nat? functions to the core.
• Add (dyn :executable) at top level to get what used to be
  (process/args 0).
• Add :linux to platforms returned by (os/which).
• Update jpm to build standalone executables. Use declare-executable for this.
• Add use macro.
• Remove process/args in favor of (dyn :args).
• Fix bug with Nanbox implementation allowing users to created
  custom values of any type with typed array and marshal modules, which
  was unsafe.
• Add janet_wrap_number_safe to API, for converting numbers to Janets
  where the number could be any 64 bit, user provided bit pattern. Certain
  NaN values (which a machine will never generate as a result of a floating
  point operation) are guarded against and converted to a default NaN value.