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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information