more options for size

[nsajko]

1. Perhaps add a new struct StaticSizeArray which would have size as a type parameter instead of a field.

2. Perhaps parameterize FixedSizeArray by the element type of the size field. I'm not sure how likely this is to be useful, but I know that GPU programs tend to avoid 64-bit values, so perhaps this would be a desired next step after support GenericMemory by making the storage type a parameter #52.