User friendly delimited continuations. While this was doable with
signals before, this does not require C and will play nicely with
existing error handling, defers, and with statements.
These additions, along with the change that user signals 0-4 cannot
be resumed, allow delimited continuation semantics, while repsecting
existing forms like `defer`, `with`, `with-vars`, etc.
This should help when redefining certain forms. Will also
not do functional arity checking against nil forms, as that
is the default value when a def doesn't evaluate.
Required a few changes to APIs, namely janet_root_fiber()
to get topmost fiber that is active in the current scheduler.
This is distinct from janet_current_fiber(), which gets the bottom
most fiber in the fiber stack - it might have a parent, and so cannot
be reliably resumed.
This is the kind of situation that makes symmetric coroutines more
attractive.
These functions interact with Janet's dynamically scoped
IO functions in a manner that is more useful the file/flush.
We can still redirect to a buffer without changing our code.
This way we can support fewer build configurations. Also, remove
all undefined behavior due to use of memcpy with NULL pointers. GCC
was exploiting this to remove NULL checks in some builds.
* I deleted Alt-H and Alt-L because Ctrl-F and Ctrl-B serve the same
roles.
* Ctrl-W, Alt-D, Alt-F, and Alt-B behave more similarly to the same
key bindings on GNU readline.
* Improved documentation of REPL keybindings on man page.
* Home and End keys now work on more terminal environments.
* Removed bindings for `Esc OH` and `Esc OF` because andrewchambers
doesn't need those bindings and the bindings don't seem to make much
sense for Home and End. `Esc O` is Single Shift Select of G3 Character
Set in xterm. https://invisible-island.net/xterm/ctlseqs/ctlseqs.html
