runtime: make iface/eface handling more type safe

Change compiler-invoked interface functions to directly take
iface/eface parameters instead of fInterface/interface{} to avoid
needing to always convert.

For the handful of functions that legitimately need to take an
interface{} parameter, add efaceOf to type-safely convert *interface{}
to *eface.

Change-Id: I8928761a12fd3c771394f36adf93d3006a9fcf39
Reviewed-on: https://go-review.googlesource.com/16166
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/export_test.go

@@ -127,7 +127,7 @@ var BigEndian = _BigEndian
 // For benchmarking.
 
 func BenchSetType(n int, x interface{}) {
-	e := *(*eface)(unsafe.Pointer(&x))
+	e := *efaceOf(&x)
 	t := e._type
 	var size uintptr
 	var p unsafe.Pointer