Trans refactor part 1

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This removes all of the explicit @mut fields from CrateContext. There are still a few that are managed, but no longer do we have @mut bool in the structure.

Most of the changes are changing @CrateContext to @mut CrateContext, though I did change as many as I could get away with to &CrateContext and &mut CrateContext. The biggest thing preventing me from changing to &[mut] in most places was the instruction counter thing. In two cases, where I got a static borrow error and a dynamic borrow error, I opted to remove the count call there as it was literally the only thing preventing me from switching to &mut CrateContext parameters in both cases.

Other things to note:

• the EncoderContext uses borrowed pointers with lifetimes, since it can, though that required me to work around the limitation of not being able to move a structure with borrowed pointers into a heap closure. I changed as much as I could to stack closures, but unfortunately I hit the AST visitor and changing that is somewhat outside the scope of this PR. Instead (and there is a comment to this effect) I choose to unsafely get the structure into the heap, this is because I know the lifetimes involved are safe, even though the compiler can't prove it.
• Many of the changes are workarounds because of the borrow checker, either dynamically freezing it for too long, or inferring too large a scope. This is mostly just from nested function calls where each borrow is considered to last for the entire statement. Other cases are where CrateContext was borrowed in a match causing it to be borrowed for the entire length of the match, even though that wasn't wanted (or needed).
• I haven't yet tested to see if this changes compilation times in any way. I doubt there will be much of an impact however, as the only major improvements are less indirection and fewer refcount bumps.
• This lays the foundations to remove many more heap allocations in trans as many cases can be changed to use lifetimes instead.

This change includes some other, minor refactorings, as I am planning a series, however I don't want to submit them all at once as it will be hell to continually rebase.

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I accidentally deleted the branch.

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@graydon @brson @pcwalton Someone please look at this, every other PR that gets merged conflicts with this one, I'd like to have this merged before I'm ready with part 2.

[graydon]

I'm on a phone today, detailed review outside scope of what I can do. I would not object to you placing r=me on it, if it does in fact do what the comments say, and if you're willing to do followups on anything I or others pick out in later review?

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@graydon that's fine. I included all the major strokes in the comments, and it seems to work locally. As I said, I am planning on a series of changes to try and modernize trans as best I can so I'm more than willing to follow up any comments.