Make Expr::MethodCall available in non-"full" mode

[dtolnay]

This appears in derive input in the case of const associated functions.

#[derive(Derive)]
pub enum E {
    V = CONFIG.compute_thing(),
}

struct Config { ... }

const CONFIG: Config = Config { ... };

impl Config {
    const fn compute_thing(&self) -> isize { ... }
}

Previously with syn in non-"full" mode, a method call expression such as CONFIG.compute_thing() would parse incorrectly as Expr::Call containing Expr::Field as the called function, rather than Expr::MethodCall containing Expr::Path as the receiver.