Use a TypedArena in ty::ctxt.

[eddyb]

This was inspired by seeing a LLVM flatline of ~600MB when running rustc with jemalloc (each type's t_box_ is allocated on the heap, creating a lot of fragmentation, which jemalloc can deal with, unlike glibc).

[nikomatsakis]

meta comment: It seems like a good idea to introduce a convention of using 'tcx for the lifetime of the type context, or something like that.

[eddyb]

@nikomatsakis I'll wait and see how much this can be reduced with better lifetime variance inference support, and only then start adding 'tcx, but I like it otherwise.

[alexcrichton]

ping, looks like this can be rebased now that the de-@ phase has landed

[eddyb]

@alexcrichton I'm waiting for a snapshot so I can make traits generic over lifetimes (#12807), otherwise this would still be hacky.

[alexcrichton]

I talked to @eddyb on IRC, and he said that this needs some more work, and to close it for now.

[eddyb]

This is plagued by "pointer has a longer lifetime than the data it references" errors, sadly - see my comment on #10396.

[thestinger]

This needs another rebase. I'm going to close it for now.

[emberian]

r=me with rebase

[pcwalton]

Do we know what the memory usage implications of this are?