Core: Add constexpr constructors to math structs

[Repiteo]

Subsection of #91992

I quickly realized the scope of adding constexpr to math structs, even at a reduced scope, made for a pretty daunting and exhaustive changelog overall. So instead, this aims to be a significantly reduced version of the original commit by focusing exclusively on constructors. In the interest of keeping the changecount as low as possible, no attempts were made to reorganize the structs from how they are now. Absolutely no other functions were made constexpr; if a constructor relied on other functions in their constructors, they simply weren't converted.

The only changes outside of constructors were:

• Removing redundant variables (would cause warnings/errors in GCC).
• Removing Projection's empty destructor.
• Changing Vector2/Vector2i's union struct layout from 2 layers of nesting to 1 (intellisense throws a fit otherwise).