proc: refactoring: merge target into proc

- moved target.Interface into proc as proc.Process
- rename proc.IThread to proc.Thread
- replaced interfaces DisassembleInfo, Continuable and
  EvalScopeConvertible with Process.
- removed superfluous Gdbserver prefix from types in the gdbserial
  backend.
- removed superfluous Core prefix from types in the core backend.

pkg/proc/stack.go

@@ -40,7 +40,7 @@ type Stackframe struct {
 
 // Stacktrace returns the stack trace for thread.
 // Note the locations in the array are return addresses not call addresses.
-func ThreadStacktrace(thread IThread, depth int) ([]Stackframe, error) {
+func ThreadStacktrace(thread Thread, depth int) ([]Stackframe, error) {
 	regs, err := thread.Registers(false)
 	if err != nil {
 		return nil, err