Write CanonicalizeCase pass and allow fall-through default cases

[rrnewton]

Right now the #lang gibbon case expressions are very fragile. They require exhaustive pattern matches and I think they even require the same order as in the data definition.

This can be relaxed by adding a pass early in the compiler pipeline which:

• puts the cases in a canonical order (if that's really needed... but in any case it doesn't hurt)
• replaces all missing cases with errors

[rrnewton]

Ah, right, it's SourceInterp that's making the lame assumption about ordering.

test25c is there to tickle this.

[rrnewton]

There's still the issue of handling incomplete pattern matches.

[ckoparkar]

There's still the issue of handling incomplete pattern matches.

This can be accomplished by an L0->L0 pass that inspects case expressions and adds appropriate MISSING_PATTERN -> error alternatives and prints a warning during compilation.