Eliminate dot in nullary-tag variant patterns

[brson]

Nobody likes it, but there's no agreement on how to fix it.

[catamorphism]

(That is, eliminate the '.' after a nullary-tag pattern in an alt)

I'll implement this, by disallowing name conflicts between variables and in-scope nullary tags.

[brson]

Uh, yeah. Terrible description. I was typing fast. I've updated the title to be more descriptive.

[catamorphism]

c3bc8fa is a transitional version that supports patterns either with or without the '.'. I haven't yet changed all the compiler and library code to not use the '.', so I'm leaving this open.

[catamorphism]

5b028f5 removes support for the '.'. HUZZAH