This patchset makes the shell capable of lazily resolving and executing
sequences of commands, to allow for putting logical sequences in the
background.
In particular, it enables And/Or/Sequence nodes to be run in the background,
and consequently unmarks them as `would_execute`.
Doing so also fixes job control to an extent, as jobs are now capable of
having 'tails', so sequences can be put in the background while
preserving their following sequences.
This commit also completely reworks the execution, highlighting and
completion model to work with the new AST.
New additions:
- $(...) stdout captures
- fd>&fd redirections
- fd>&- redirections (close fd)
- read-write redirections (<> path)
- completely event-based execution
- the weird idea of allowing the user to redirect the shell's own fds
- variables in strings
- local variables
- minimal list support
- adding hyperlinks to all paths that exist
