Add support for simple Literal fields

[lebrice]

This adds support for simple fields, like:

from typing import Literal
from dataclasses import dataclass

@dataclass
class Config:
    model: Literal["resnet18", "resnet50"] = "resnet18"

Also adds support for:

• Literal[ints]
• Literal[strings]
• Literal[enums]
• Literal[bools]

Doesn't yet add support for:

• Optional[Literal[X]]
• List[Literal[X]]
• Tuple[Literal[X], ...]

Signed-off-by: Fabrice Normandin fabrice.normandin@gmail.com