feature model interface (#222)

* make variable declaration more concise

* add model interface

* optimize interface methods

* fix: go test failed

* warp returns

* optimize

tools/goctl/model/sql/template/new.go

@@ -1,8 +1,8 @@
 package template
 
 var New = `
-func New{{.upperStartCamelObject}}Model(conn sqlx.SqlConn{{if .withCache}}, c cache.CacheConf{{end}}) *{{.upperStartCamelObject}}Model {
-	return &{{.upperStartCamelObject}}Model{
+func New{{.upperStartCamelObject}}Model(conn sqlx.SqlConn{{if .withCache}}, c cache.CacheConf{{end}}) {{.upperStartCamelObject}}Model {
+	return &default{{.upperStartCamelObject}}Model{
 		{{if .withCache}}CachedConn: sqlc.NewConn(conn, c){{else}}conn:conn{{end}},
 		table:      "{{.table}}",
 	}