Defer reasoning about region relationships until after regionck.

This patch makes error handling for region inference failures more
uniform by not reporting *any* region errors until the reigon inference
step. This requires threading through more information about what
caused a region constraint, so that we can still give informative
error messages.

I have only taken partial advantage of this information: when region
inference fails, we still report the same error we always did, despite
the fact that we now know precisely what caused the various constriants
and what the region variable represents, which we did not know before.

This change is required not only to improve error messages but
because the region hierarchy is not in fact fully known until regionck,
because it is not clear where closure bodies fit in (our current
treatment is unsound). Moreover, the relationships between free variables
cannot be fully determined until type inference is otherwise complete.

cc #3238.
Correct handling of if/match, and make explicit computation of
common supertypes.

This was breaking with the change to regions because of the
(now incorrect) assumpton that our inference code makes,
which is that if a <: b succeeds, there is no need to compute
the LUB/GLB.
move docs into doc.rs
convert region_inference into a module, so I can attach docs
Move existing docs into doc.rs
Adjust documentation to describe how closures and closure bounds
affect things.
auto merge of #7187 : nikomatsakis/rust/issue-3238-defer-reasoning-about-regions, r=pcwalton

This addresses issue #3238. It will also (in the future) enable more specific error messages for region inference failures, though I don't do much with that in this commit.

See first commit for details and brief motivation. 

You may want to begin with the last commit, which adds some documentation explaining in a bit more detail some of the issues that arise when trying to handle closures. The docs include some material on how inference will handle closure bounds, though that code is not yet written (hoping to tackle that next).

r? @pcwalton (or anyone, but I guess you are the most versed in the code in question)