Janet 1.23.0

This release brings a number of new features that will make Janet useful in even more domains, including mutexes and reader-writer locks to the ev module to improve interactions with multi-threaded, native code, better named arguments support, and an experimental FFI module. Also a number of improvements and simplification to Janet's debugging capabilities have been made, and exposed with the new debugger function.

• Add experimental ffi/ module for interfacing with dynamic libraries and raw function pointers. Only available
  on 64 bit linux, mac, and bsd systems.
• Allow using &named in function prototypes for named arguments. This is a more ergonomic
  variant of &keys that isn't as redundant, more self documenting, and allows extension to
  things like default arguments.
• Add delay macro for lazy evaluate-and-save thunks.
• Remove pthread.h from janet.h for easier includes.
• Add debugger - an easy to use debugger function that just takes a fiber.
• dofile will now start a debugger on errors if the environment it is passed has :debug set.
• Add debugger-on-status function, which can be passed to run-context to start a debugger on
  abnormal fiber signals.
• Allow running scripts with the -d flag to use the built-in debugger on errors and breakpoints.
• Add mutexes (locks) and reader-writer locks to ev module for thread coordination.
• Add parse-all as a generalization of the parse function.
• Add os/cpu-count to get the number of available processors on a machine