Implement setref

[andychu]

Strictly speaking this doesn't need to make the 2020 cut, but it's a better version of bash's named references.

Need to decide on the declaration syntax:

proc p (s, :out) {
}

 or

proc p (s, out Ref) {
}

There is a similar issue iwth blocks

proc p (&b) {
}

or

proc p (b Block) {
}

[andychu]

This can build on the work in #600

[andychu]

Probably essential for a better completion API, a better prompt hook, and a better $PS4, etc.

[andychu]

The thing that would make this not essential is if we have an optimized command sub: #799

So then we can "return" values from procs in a different way

[andychu]

Fully done 0.8.11 with http://www.oilshell.org/release/latest/doc/variables.html

[andychu]

https://www.oilshell.org/blog/2021/07/release-0.8.12.html