Allow const function calls of various methods

[alice-i-cecile]

This is very useful for specifying configuration. For example, in Bevy I'm trying to write:

    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 :(

[bitshifter]

Closing as duplicate of #76. There's discussion in there as to why this is the case and what needs to happen to resolve it.

[bitshifter]

There are macros to create const glam types, Transform is a bevy type so that's a bit trickier. It contains a Quat and Quat::new cannot be made const fn because it uses SSE __m128 for storage and there is no const fn to initialise one of those. That is the problem, this needs to be addressed in Rust's std::arch. Bevy could create a const_transform! macro using the glam ones in the interim. You can make a const Vec2 using glam::const_vec2!.