This comes with a terminology change. All linkage-symbols are 'extern'
now, including rust syms in other crates. Some extern ABIs are
merely "foreign". The term "native" is retired, not clear/useful.
What was "crust" is now "extern" applied to a _definition_. This
is a bit of an overloading, but should be unambiguous: it means
that the definition should be made available to some non-rust ABI.
Some code that handles unary and binary exprs' callee IDs was
forgetting to handle the index expr case (since calls to
user-defined index operators also have callee IDs). This was
manifesting as an ICE in trans because when monomorphizing a
function that had one of these operators in it (an index into a
dvec, in the test case), the callee ID would be unbound to a type.
Fixed it. Closes#2631.
