cmd/compile: add Type.Elem

This removes almost all direct access to
Type’s heavily overloaded Type field.

Mostly generated by eg, manually checked.

Significant manual changes:

* reflect.go's typPkg used Type indiscriminately.
  Use it only for specific etypes.
* gen.go's visitComponents contained a usage of Type
  with structs. Using Type for structs no longer
  occurs, and the Fatal contained therein has not triggered,
  so it has been axed.
* Scary code in cgen.go's cgen_slice is now explicitly scary.

Passes toolstash -cmp.

Change-Id: I2dbfb3c959da7ae239f964d83898c204affcabc6
Reviewed-on: https://go-review.googlesource.com/21331
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/closure.go

@@ -490,7 +490,7 @@ func makepartialcall(fn *Node, t0 *Type, meth *Sym) *Node {
 	}
 	basetype := rcvrtype
 	if Isptr[rcvrtype.Etype] {
-		basetype = basetype.Type
+		basetype = basetype.Elem()
 	}
 	if basetype.Etype != TINTER && basetype.Sym == nil {
 		Fatalf("missing base type for %v", rcvrtype)