janet: Update to 1.13.1

## 1.13.1 - 2020-12-13
- Pretty printing a table with a prototype will look for `:_name` instead of `:name`
  in the prototype table to tag the output.
- `match` macro implementation changed to be tail recursive.
- Adds a :preload loader which allows one to manually put things into `module/cache`.
- Add `buffer/push` function.
- Backtick delimited strings and buffers are now reindented based on the column of the
  opening delimiter. Whitespace in columns to the left of the starting column is ignored unless
  there are non-space/non-newline characters in that region, in which case the old behavior is preserved.
- Argument to `(error)` combinator in PEGs is now optional.
- Add `(line)` and `(column)` combinators to PEGs to capture source line and column.
  This should make error reporting a bit easier.
- Add `merge-module` to core.
- During installation and release, merge janetconf.h into janet.h for easier install.
- Add `upscope` special form.
- `os/execute` and `os/spawn` can take streams for redirecting IO.
- Add `:parser` and `:read` parameters to `run-context`.
- Add `os/open` if ev is enabled.
- Add `os/pipe` if ev is enabled.
- Add `janet_thread_current(void)` to C API
- Add integer parsing forms to pegs. This makes parsing many binary protocols easier.
- Lots of updates to networking code - now can use epoll (or poll) on linux and IOCP on windows.
- Add `ev/` module. This exposes a fiber scheduler, queues, timeouts, and other functionality to users
  for single threaded cooperative scheduling and asynchronous IO.
- Add `net/accept-loop` and `net/listen`. These functions break down `net/server` into it's essential parts
  and are more flexible. They also allow further improvements to these utility functions.
- Various small bug fixes.

(nia)