Track record fields in typestate

[catamorphism]

It seems harmless to also allow typestate constraints to mention immutable fields of records -- for example, letting you write:

check type_has_static_size(cx.tcx, t);

instead of

let tcx = cx.tcx;
check type_has_static_size(tcx, t);

[graydon]

Significantly more complex. I'd recommend against it.

[catamorphism]

I now agree.