Log Message:
janet: Update to 1.15.2

## 1.15.2 - 2021-02-15
- Fix bug in windows version of `os/spawn` and `os/execute` with setting \ 
environment variables.
- Fix documentation typos.
- Fix peg integer reading combinators when used with capture tags.

## 1.15.0 - 2021-02-08
- Fix `gtim` and `ltim` bytecode instructions on non-integer values.
- Clean up output of flychecking to be the same as the repl.
- Change behavior of `debug/stacktrace` with a nil error value.
- Add optional argument to `parser/produce`.
- Add `no-core` option to creating standalone binaries to make execution faster.
- Fix bug where a buffer overflow could be confused with an out of memory error.
- Change error output to `file:line:column: message`. Column is in bytes - tabs
  are considered to have width 1 (instead of 8).