reflect: let reflect.Type be of interface type

This matches the main Go implementation and (among others) fixes a
compatibility issue with the encoding/json package. The encoding/json
package compares reflect.Type variables against nil, which does not work
as long as reflect.Type is of integer type.

This also adds a reflect.RawType() function (like reflect.Type()) that
makes it easier to avoid working with interfaces in the runtime package.
It is internal only, but exported to let the runtime package use it.

This change introduces a small code size increase when working with the
reflect package, but I've tried to keep it to a minimum. Most programs
that don't make extensive use of the reflect package (and don't use
package like fmt) should not be impacted by this.

interp/interpreter.go

@@ -277,7 +277,7 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 				copy(dstBuf.buf[dst.offset():dst.offset()+nBytes], srcBuf.buf[src.offset():])
 				dstObj.buffer = dstBuf
 				mem.put(dst.index(), dstObj)
-			case callFn.name == "(reflect.Type).Elem":
+			case callFn.name == "(reflect.rawType).elem":
 				if r.debug {
 					fmt.Fprintln(os.Stderr, indent+"call (reflect.rawType).elem:", operands[1:])
 				}