Add unaligned ByteArray# access primops

I would like unaligned store and unaligned load primops in GHC.Prim. Specifically: for X in {Int,Int16,Int32,Int64,Word,Word16,Word32,Word64,Float,Double,WideChar,Addr,StablePtr} and where X# is the corresponding Int#/Word#/Float#/Double#/Char#/Addr#/StablePtr# type, I'd like the following primitives:

-- | Reads an 'X#', offset in bytes
indexXArrayOffBytes# :: ByteArray# -> Int# -> X#
-- | Reads an 'X#', offset in bytes
readXArrayOffBytes# :: MutableByteArray# s -> Int# -> State# s -> (#State# s, X# #)
-- | Writes an 'X#', offset in bytes
writeXArrayOffBytes# :: MutableByteArray# s -> Int# -> X# -> State# s -> State# s

Note that the indexXOffAddr# family of functions already provides unaligned loads and similarly writeXOffAddr# provides unaligned stores, so GHC's code generator must handle these already. For example, we could use these to provide unaligned loads for pinned ByteArray#s:

indexWord16ArrayOffBytes# :: ByteArray# -> Int# -> Word#
indexWord16ArrayOffBytes# arr offset = 
  indexWord16OffAddr# (byteArrayContents# arr `plusAddr#` offset) 0#

Trac metadata

Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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