Mark constructors for Time, Vector2, Vector3 as const

[Kolsky]

No description provided.

[crumblingstatue]

Vector2 and Vector3 new methods are already const on git master.
Time is more difficult because it's an opaque type and the constructors call into FFI.

[crumblingstatue]

Time::microseconds and Time::milliseconds are const fn, but making Time::seconds const fn isn't possible right now, because of rust-lang/rust#57241

[crumblingstatue]

I think these are now as reasonably const as possible, so I'm closing this.