Simply service typing: part1

fix #103

a service is encoded by
```
type ('get,'post,+'meth,+'attached,+'kind,+'tipo,+'gn,+'pn,+'reg,+'ret) service
```
(new) `'meth` : http method ('Get, 'Post, 'Put, 'Delete)
(new) `'attached` : `['Attached of a_s | 'Nonattached of na_s ]`
(updated) `'kind` : `['Service | 'AttachedCoservice | 'NonattachedCoservice | 'External ]`