Allow const function calls of various methods

This is very useful for specifying configuration. For example, in Bevy I'm trying to write:

```rust
    pub const PADDLE_STARTING_LOCATION: Transform = Transform::from_xyz(0.0, -215.0, 0.0);
    pub const PADDLE_SIZE: Vec2 = Vec2::new(120.0, 30.0);
```
but neither of these are const fn's :(