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