internal/codegen: add Enum.Valid and AllEnumValues

This commit adds a Valid method for enum types
and an All...Values function to get all enum values.

This makes it easier to work with enums.

Fixes #1607

Author: josharian

examples/batch/postgresql/models.go

@@ -86,6 +86,18 @@ func (e *{{.Name}}) Scan(src interface{}) error {
 	}
 	return nil
 }
+
+func (e {{.Name}}) Valid() bool {
+  switch e {
+  case {{ range $idx, $name := .Constants}}{{ if gt $idx 0 }},{{ end }}{{.Name}}{{ end }}:
+    return true
+  }
+  return false
+}
+
+func All{{ .Name }}Values() []{{ .Name }} {
+	return []{{ .Name }}{ {{ range .Constants}}{{ .Name }}, {{ end }} }
+}
 {{end}}
 
 {{range .Structs}}