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/plive.go

@@ -932,7 +932,7 @@ func onebitwalktype1(t *Type, xoffset *int64, bv Bvec) {
 			*xoffset += t.Width
 		} else {
 			for i := int64(0); i < t.Bound; i++ {
-				onebitwalktype1(t.Type, xoffset, bv)
+				onebitwalktype1(t.Elem(), xoffset, bv)
 			}
 		}