builtin: use type aliases for rune and byte

As motivated by https://golang.org/design/18130-type-alias which says:

https://github.com/golang/proposal/blob/master/design/18130-type-alias.md#relationship-to-byte-and-rune

> The language specification already defines byte as an alias for
> uint8 and similarly rune as an alias for int32, using the word alias
> as an informal term. It is a goal that the new type declaration
> semantics not introduce a different meaning for alias. That is, it
> should be possible to describe the existing meanings of byte and
> uint8 by saying that they behave as if predefined by:
>
>     type byte = uint8
>     type rune = int32

So, do that. Seems to work.

Updates #18130

Change-Id: I0740bab3f8fb23e946f3542fdbe819007a99465a
Reviewed-on: https://go-review.googlesource.com/45017
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: bradfitz

src/builtin/builtin.go

@@ -85,11 +85,11 @@ type uintptr uintptr
 // byte is an alias for uint8 and is equivalent to uint8 in all ways. It is
 // used, by convention, to distinguish byte values from 8-bit unsigned
 // integer values.
-type byte byte
+type byte = uint8
 
 // rune is an alias for int32 and is equivalent to int32 in all ways. It is
 // used, by convention, to distinguish character values from integer values.
-type rune rune
+type rune = int32
 
 // iota is a predeclared identifier representing the untyped integer ordinal
 // number of the current const specification in a (usually parenthesized)