Add SmallArray# type

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.

Trac metadata

Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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