Janet 1.5.0

This release brings a lot of updates to Janet, with some small, potentially backwards incompatible changes.

Changelog:

• os/date now defaults to UTC.
• Add --test flag to jpm to test libraries on installation.
• Add math/rng, math/rng-int, and math/rng-uniform.
• Add in function to index in a stricter manner. Conversely, get will
  now not throw errors on bad keys.
• Indexed types and byte sequences will now error when indexed out of range or
  with bad keys.
• Add rng functions to Janet. This also replaces the RNG behind math/random
  and math/seedrandom with a consistent, platform independent RNG.
• Add with-vars macro.
• Add the quickbin command to jpm.
• Create shell.c when making the amalgamated source. This can be compiled with
  janet.c to make the janet interpreter.
• Add cli-main function to the core, which invokes Janet's CLI interface.
  This basically moves what was init.janet into boot.janet.
• Improve flychecking, and fix flyching bugs introduced in 1.4.0.
• Add prin, eprint, eprintf and eprin functions. The
  functions prefix with e print to (dyn :err stderr)
• Print family of functions can now also print to buffers
  (before, they could only print to files.) Output can also
  be completely disabled with (setdyn :out false).
• printf is now a c function for optimizations in the case
  of printing to buffers.