Implicit tag statement

Add an library-builder API to register a new tag as a statement (so it never goes out of scope). Would share an API with the `implicit_connect` blocks.

```python
self.add_tag(this_port, [Common])
```

Required for #1 

Note: currently, the only way to register a tag is temporarily through the scoped (blocked) `implicit_connect` blocks. Which obviously won't work for ground declarations.