Import fresh ssa sources for go1.4
First attempt to merge gub 1.2 changes into 1.4:
package name ssa -> ssa2 to avoid namespace conflicts
Register made global
start/end positions added to various Syntactic objects like parameters and, constants,
Interpreter minimally doing something. A couple of tests even pass, although many more fail.

ssa.go:
 * allow look a local by name,
 * add Function breakpoint
 * add Scope to basic block and Function

dom.go:
  * make dom.Children public and ckeep track of level of node within dominator tree

interp:
  * golang/x/tools/go/ssa -> github.com/rocky/ssa-interp
  * interp.Frame is public
  * interp.Value is public
  * interp.Struct is public is no longer an array but now includes field names

interp/external.go:
  * remove functions defined in external4gub and external_math

interp/interp.go:
  * interpreter, i, is now global, interpreter Mode is public
  * add instruction tracing flags TraceMode, TraceEvent
  * add ability to track goroutiens (goTops, nGoroutines)
  * Better Position tracking by including *ends* as well as starts
DebugRef.Object is public
We had used bogus interp.Array when we should have had interp.Structure. address
some of those issues. Some other probably still remain.
interp.go:
*  visitInstr() save instruction to pass off to debugger
*  save ssa2.DebugRef ifnormation
*  panic uses source Panic not interpreter panic
*  fix wrong Goroutine parameter in go() call.
*  index check channel index
*  index check array index

reflect.go:
   fill in reflect-value Structure properly
builder.go: add trace instructions for the debugger to be able to stop at
stepping events. Still need to add Scope info in blocks
interp/interp_test.go: reinstate $GOROOT/src tests
interp/reflect.go: correct one more bogus Array use.
Work in Progress - debugger finally will compile and tortoise can call that. Up next, fill in scopes.
Add scope information and more precise location back form old interpreter forka
Update branch information.
Merge branch 'go1.4'

Conflicts:
	interp/value.go
Fix cmd Makefile target