Don't show artificial variables in stack traces

[girving]

When we lift anonymous functions into toplevel function declarations, we call them all 'f'. This name should not show up in error messages or stack traces:

base:180:13-39:
  'fold base.f (List Void) (List Int)' does not match any overloads, tried:

The message should make it clear that we're talking about an anonymous function and give the SrcLoc.