Rename the nullability ivar.
Merge pull request #71 from robrix/nullable-ivar

Rename the nullability ivar.
Simplify reducing reductions.
Infer reductions’ function description if nil.
Compact the reduction of the empty parser.
Propagate function descriptions when compacting.
Preserve function descriptions when deriving.
Infer function descriptions when composing.
Move composition descriptions into a temporary.
Break composition up for debugging.
Merge pull request #72 from robrix/tidier-reduction

Tidier reduction
The empty pair is described as ().
Weird and unpleasant improvement to parse trees.

Introduces null delays, which are delays of HMRNull, to avoid eagerly
reducing and evaluating effects with reductions inside concatenations.
Merge branch 'master' into lexer

Conflicts:
	Hammer/HMRReduction.m