Log Message:
janet: Update to 1.15.4

## 1.15.4 - 2021-03-16
- Increase default nesting depth of pretty printing to `JANET_RECURSION_GUARD`
- Update meson.build
- Add option to automatically add shebang line in installed scripts with `jpm`.
- Add `partition-by` and `group-by` to the core.
- Sort keys in pretty printing output.

## 1.15.3 - 2021-02-28
- Fix a fiber bug that occured in deeply nested fibers
- Add `unref` combinator to pegs.
- Small docstring changes.