Use `pg.typing` as type annotations

[daiyip]

PyGlove provides a runtime typing feature through pg.typing, for example, a symbolic class can be created via:

from dataclasses import dataclass
import pyglove as pg

@pg.symbolize([
  ('x', pg.typing.Int(min_value=1, max_value=2)),
  ('y', pg.typing.Callable([pg.typing.Int(max_value=0)], returns=pg.typing.Bool))
])
@dataclass
def Foo:
    x: int
    y: Callable[[int], bool]

There is a redundancy in this definition. Ideally, users should be able to do:

@pg.symbolize
@dataclass
def Foo:
   x: pg.typing.Int(min_value=1, max_value=2)
   y: pg.typing.Callable([pg.typing.Int(max_value=0)], returns=pg.typing.Bool())

And this code should work for both static type check (PyType) and runtime type check.

Additionally, we hope x: int can be recognized by PyGlove as a shortcut to x: pg.typing.Int().