Command sub-invocations

[brechtcs]

In the original specifications for Webterm, Paul proposed the following syntax for command sub-invocations:

# basic usage
> change-case -u (hello-world)
HELLO, WORLD!

# nested invocations
> concat (change-case -u (hello-world)) (change-case -l (hello-world))
HELLO, WORLD! hello, world!

In dterm I decided to prioritise support for iterators, which makes this slightly less straigtforward. It's still on the roadmap though, just have to figure how cases like this are handled in Bash for example.