JS: copyMutableByteArray# is probably wrong
I noticed while working on !9629 that StgToJS appears to generate code for CopyMutableByteArrayOp
that copies the bytes one-by-one from back to front. This is wrong if a user makes a call like copyMutableByteArray# marr# 32# marr# 0# 96#
, because some locations will be written to as destination bytes before they are read from as source bytes.