Auto merge of #5432 - djc:split-context, r=alexcrichton

Auto merge of #5432 - djc:split-context, r=alexcrichton

Split Context into two types

@matklad here's the crazy attempt to make the `Context` situation slightly more sane. It basically splits all of the stuff that is immutable over the lifetime of the compilation from the stuff that's initialized empty. The latter is now stored in a type which I have provisionally called `BuildProgress` -- feel free to bikeshed about the name (I came up with `BuildData` as another alternative). It stores a reference to the `Context` to make things easier for now, we might want to change that down the line.

IMO this is a pretty good improvement, which clarifies the design quite a bit.

@alexcrichton you'll probably want to review this, too.