Add a SmallArray# (and SmallMutableArray#) type that doesn't have a card table. This would

• save some memory (two words per array),
• make updates cheaper (no writing to the card table), and
• make GC faster (no traversing the card table).

The use case is the unordered-containers package, which uses small arrays to implement a hash array mapped trie.

A starting point would be [changeset:0417404f/ghc], which added the card table to the current array type.