Fixes #2806
Fixes #3197
Fixes #3138

There is also code for ~Trait and &Trait, but these are currently (incorrectly)
synonyms for "as @Trait" and "as &Trait".

(Also "interface" -> "trait" in error messages.)

Exprs that could be applications of overloaded operators
(expr_unary, expr_binary, expr_index) relied on the previous node ID
being "reserved" to carry extra typechecking info. This was
incredibly error-prone. Fixed it; now all exprs have two node IDs
(which will be wasted in some cases; future work could make this
an option instead if the extra int field ends up being a performance
problem).

Closes #2804

And change structurally_resolved_type() to force resolution of
non-region variables.