Document effects/handlers

[jamescheney]

The recently-added extension to support effects and handlers still needs to be documented. The wiki page here:

https://github.com/links-lang/links/wiki/Effects-and-Handlers

should be filled in with at least:

• explanation of how to activate effects/handlers (e.g. command line option, REPL settings or config settings)
• an example or two (perhaps copied from the test suite / examples)
• an explanation of the main constructs and intended meaning
• (perhaps) pointers to related papers

[jamescheney]

I suggest we defer this to 0.7.3 and release 0.7.2 ASAP.

[dhil]

I am somewhat reluctant to document the handlers extension until I have finished the implementation of computation patterns, which is going to completely change the surface syntax for handler cases.

[jamescheney]

Fine with me, just means no one besides you will try using handlers, which maybe is a feature rather than bug if it's all about to change.