implement LDW operation

Add a memory setOcta helper to set 8 memory cells aligned on an octabyte.
Add an address utility that converts an long to an octabyte hex string
for standard memory address keys in the memory map.
The test for LDW is from Knuth's MMIX pdf.
correct sign extending and memory alignment in LDW

Add tests from Knuth's example where the data loaded from memory has
a high bit of 1 and properly sign extend before putting in register.
Also fix memory alignment code for same test.  LDW Memory2[1005]
should load bytes 1004 and 1005, not 1005 and 1006.