Use value.TypeString in TypeAssertion.String

The value.TypeString function is what the rest of the package uses
and is slightly cleaner than using reflect.Type.String.

Updates #305

cmp/path.go

@@ -284,7 +284,7 @@ type typeAssertion struct {
 
 func (ta TypeAssertion) Type() reflect.Type             { return ta.typ }
 func (ta TypeAssertion) Values() (vx, vy reflect.Value) { return ta.vx, ta.vy }
-func (ta TypeAssertion) String() string                 { return fmt.Sprintf(".(%v)", ta.typ) }
+func (ta TypeAssertion) String() string                 { return fmt.Sprintf(".(%v)", value.TypeString(ta.typ, false)) }
 
 // Transform is a transformation from the parent type to the current type.
 type Transform struct{ *transform }