Create int2array and array2int

These convert between LLVM integers and dat_t arrays.  The general
idea here is that I want to be able to reuse these for loading from
memories.
Implement RD and WR

These are the two memory nodes that I actually know how to generate
from Chisel.  The other two (LD and ST) don't appear to be generated
from a Chisel Mem, so I'm just not going to touch them...

There's also a simple test case for memories in here.  Presumably a
number of the tutorial examples will now run, but I haven't tried any
yet.