Types refactor

[jamescheney]

Collapse the type structure into a single monolithic OCaml data type.

[dhil]

I have written up an initial draft proposal for the new type structure, c.f. https://gist.github.com/dhil/97711b8c42f1a2fed91b3b6a1398783d, intended to serve as a starting point for discussion.

[dhil]

For this work I have created a new branch on this repository. It is called types-refactor.

[jamescheney]

Was this closed by #859? If not can someone add a description of what remains to be done?

[slindley]

The first phase of this was done and merged in #859 , but I've left the issue open because there's more to do. What remains includes tidying up (e.g. by being more consistent about the use of unboxed rows in certain places) and removing the need for the invariant that Var, Recursive, and Closed constructors can only occur inside Meta constructors.

[jamescheney]

At today's meeting we agreed @slindley would add a checklist for remaining requirements for completion of this issue for 0.9.2.

[jamescheney]

This is milestoned 0.9.2, which we hoped to get released around now. Is it likely to be resolved in the next week or so, or should we postpone it?

(0.9.2 is needed as a bugfix release due to #807 )

[dhil]

It is not crucial for the bugfix release. Postponed.