Log Message:
janet: Update to 1.12.2

## 1.12.2 - 2020-09-20
- Add janet\_try and janet\_restore to C API.
- Fix `os/execute` regression on windows.
- Add :pipe option to `os/spawn`.
- Fix docstring typos.

## 1.12.1 - 2020-09-07
- Make `zero?`, `one?`, `pos?`, and `neg?` polymorphic.
- Add C++ support to jpm and improve C++ interop in janet.h.
- Add `%t` formatter to `printf`, `string/format`, and other formatter functions.
- Expose `janet_cfuns_prefix` in C API.
- Add `os/proc-wait` and `os/proc-kill` for interacting with processes.
- Add `janet_getjfile` to C API.
- Allow redirection of stdin, stdout, and stderr by passing keywords in the env \ 
table in `os/spawn` and `os/execute`.
- Add `os/spawn` to get a core/process back instead of an exit code as in \ 
`os/execute`.
  When called like this, `os/execute` returns immediately.
- Add `:x` flag to os/execute to raise error when exit code is non-zero.
- Don't run `main` when flychecking.
- Add `:n` flag to `file/open` to raise an error if file cannot be opened.
- Fix import macro to not try and coerce everything to a string.
- Allow passing a second argument to `disasm`.
- Add `cancel`. Resumes a fiber but makes it immediately error at the yield point.
- Allow multi-line paste into built in repl.
- Add `(curenv)`.
- Change `net/read`, `net/chunk`, and `net/write` to raise errors in the case of \ 
failures.
- Add `janet_continue_signal` to C API. This indirectly enables C functions that \ 
yield to the event loop
  to raise errors or other signals.
- Update meson build script to fix bug on Debian's version of meson
- Add `xprint`, `xprin`, `xprintf`, and `xprinf`.
- `net/write` now raises an error message if write fails.
- Fix issue with SIGPIPE on macOS and BSDs.