Support fetch using interface

[zsh1995]

Hi! Is it possible to provide a Getter interface for fetch ops?

type Getter interface {
    Get(key string) (interface{}, bool)
}

Env = map[string]interface{}{
    "ctx": getter,
}

Then expression can be more simple

ctx.value_want_to_get 

instead of using function

get_value("value_want_to_get")