memarg encoding about multi-memory

[rossberg]

My plan was to be as conservative as is possible without wasting space, which means allocating bits in the memarg from the high end of a single byte LEB. That would be the 7th bit (bit 6).

[AndrewScheidecker]

I think it's unlikely we'll ever need the 5th and 6th alignment bits, but I suppose it makes sense to keep them until there's a specific use for them.