spec: struct tags must be identical for types to be identical

We didn't mention this explicitly during our discussions,
but I think it fits the "identical types are spelled identically"
rule that we used.

R=gri, iant, ken2, r, rsc1
CC=golang-dev
https://golang.org/cl/1698043

doc/go_spec.html

@@ -1263,7 +1263,8 @@ literal structure and corresponding components have identical types. In detail:
 	<li>Two slice types are identical if they have identical element types.</li>
 
 	<li>Two struct types are identical if they have the same sequence of fields,
-	    and if corresponding fields have the same names and identical types.
+	    and if corresponding fields have the same names, and identical types,
+	    and identical tags.
 	    Two anonymous fields are considered to have the same name. Lower-case field
 	    names from different packages are always different.</li>