Log Message:
janet: Update to 1.9.1

## 1.9.1 - 2020-05-12
- Add :prefix option to declare-source
- Re-enable minimal builds with the debugger.
- Add several flags for configuring Janet on different platforms.
- Fix broken meson build from 1.9.0 and add meson to CI.
- Fix compilation issue when nanboxing is disabled.

## 1.9.0 - 2020-05-10
- Add `:ldflags` option to many jpm declare functions.
- Add `errorf` to core.
- Add `lenprefix` combinator to PEGs.
- Add `%M`, `%m`, `%N`, and `%n` formatters to formatting functions. These are the
  same as `%Q`, `%q`, `%P`, and `%p`, but will not truncate long values.
- Add `fiber/root`.
- Add beta `net/` module to core for socket based networking.
- Add the `parse` function to parse strings of source code more conveniently.
- Add `jpm rule-tree` subcommand.
- Add `--offline` flag to jpm to force use of the cache.
- Allow sending pointers and C functions across threads via `thread/send`.
- Fix bug in `getline`.
- Add `sh-rule` and `sh-phony` to jpm's dialect of Janet.
- Change C api's `janet_formatb` -> `janet_formatbv`, and add new function \ 
`janet_formatb` to C api.
- Add `edefer` macro to core.
- A struct/table literal/constructor with duplicate keys will use the last value \ 
given.
  Previously, this was inconsistent between tables and structs, literals and \ 
constructor functions.
- Add debugger to core. The debugger functions are only available
  in a debug repl, and are prefixed by a `.`.
- Add `sort-by` and `sorted-by` to core.
- Support UTF-8 escapes in strings via `\uXXXX` or `\UXXXXXX`.
- Add `math/erf`
- Add `math/erfc`
- Add `math/log1p`
- Add `math/next`
- Add os/umask
- Add os/perm-int
- Add os/perm-string
- Add :int-permissions option for os/stat.
- Add `jpm repl` subcommand, as well as `post-deps` macro in project.janet files.
- Various bug fixes.