Two SSA IR changes

1) Add a reference to the enclosing BasicBlock for each instruction

2) Rename the basic block instruction vector and expose it to users as a
list.  We only keep it as a vector for faster access to phi and
terminator instructions (via slicing).
Remove an obsolete comment
Add a helper to get the terminator instruction for a block
Tighten some version bounds
Add another basic block helper

This splits a basic block into phi nodes and the rest of the instructions.
Add back-links from Methods to their Class
Allow runDX to accept multiple Java source files

Also expose this helper
Switch to wl-pprint for the Pretty class