Choose external names more predictably

Choose external names more predictably

Now, for a compiler-generated binding "x", if "x" is referred to by
the exported "f", then it will be named "f_x" rather than something
like "x23".  This means that hopefully

  - compilation will more often product the same results given the
    same input (the choice of names is not dependent on the
    non-deterministic order of bindings within the compiler).

  - less recompilation will be necessary after making changes

  - navigating Core might be a bit easier.

unfortunately, compilation with -O still does not consistently produce
the same ABI.  The simplifier sometimes does different things,
apparently.

Names will be longer, but I can't see a way around that.