Forms

Would be nice to support the default HTML form system for a nicer developer experience. #9 Should probably get finished before this, though.

Hypothetical syntax:

```py
@get("/")
async def my_page():
    return "..."

@dataclass()
class Something:
    a: str
    b: str

@my_page.form(Something)
async def my_page(my_form_data: Something):
    return "..."
```