Make Vec3A::from_vec4 const

[herabit]

All of the implementations of Vec3A::from_vec4 do not seem to require non-const functionality, so marking them all as const fn will increase their versatility for API consumers.

I would make a pull request if I was more familiar with contributing to open source, as this seems like a minor change, though yeah I'd need to look into how to, do that.

[herabit]

Wish I had read the contributing guide, I'll make this into a discussion. I'll leave this open for now just in case this is the preferred channel for communication. Close this if it you desire as, yeah I will create the discussion.

[bitshifter]

If it's any consolation almost no one reads the contributing guide :) I really need to make an issue template directing people to discussions. I replied to your discussion thread but I'll leave this open until the PR is merged.

[bitshifter]

It turns out this is not currently possible because Vec3::from_vec4 invokes deref to access the x,y & z values which can't be done in a const context.

At some point this might be possible rust-lang/rust#67792.