Dedicated syntax for `Annotated`

[blhsing (Ben Hsing)]

@doerwalter

Or maybe we could use + as the operator for adding annotations to types.

I.e.

foobar: Annotated[int, A(), B()] | Annotated[str, C(), D()]

would become:

foobar: int + A() + B()] | str + C() + D()

The precedences for the + and | operator would be exactly what we need.

Using + may cause ambiguity when the type on the left is a forward reference specified with a string, e.g.:

movies: Annotated['list[Movie]', 'list of movies']

would become:

movies: 'list[Movie]' + 'list of movies'

@ on the other hand is rarely overloaded so it’s safer to repurpose.