sizeofByteArray# returns allocated words, not requested length in bytes
A byte array allocated with (
GHC.Prim.newByteArray# 7) will report it's size as '8' - that is, the stored size in
StgArrWords is the number of allocated words, not the number of requested bytes.
This menas that if I want to a
MutableByteArray# as an array type, I need a separate length fields.