[DSLX:BC] Fix disagreement between typechecker and interpreter on whether trace returns its operand

[cdleary]

Previously the bytecode interpreter thought it returned like trace_fmt! does, but historically trace! acted as an identity function so you could easily wrap it around any given subexpression, like:

let binding = f(a * b + c);

instrumenting the interior subexpression like so:

let binding = f(trace!(a*b)+c);

The fact they disagreed led to typecheck flagging a program through but then getting a bytecode interpreter error in the bowels of InterpValue.

Note that trace_fmt! and trace! are different in that the latter requires and operand but the former can just take a format string and zero operand args, so they are implemented as two bytecode ops that call into common functionality.