go.tools/go/types: cleanup: more consistent exported predicate names

Renamed predicates:
IsIdentical -> Identical
IsAssignableTo -> AssignableTo
Signature.IsVariadic -> Signature.Variadic
Object.IsExported -> Object.Exported

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/53370043
37 files changed