Space improvements

[patrickdoc]

The half edge type benefits greatly from unpacking on smaller types.

data Head a b = Head !a !b
data SmallHead = SmallHead !Char !Int

Due to -funbox-small-strict-fields, SmallHead will be 3 words, while Head Char Int will be 7. This benefit falls off for larger types, but should be exploited on common node and edge labels