Allows for things such as:

auto rect = Common::Rectangle{0, 0, 0, 0};

as opposed to being required to explicitly write out the underlying
type, such as:

auto rect = Common::Rectangle<int>{0, 0, 0, 0};

The only requirement for the deduction is that all constructor arguments
be the same type.

These types are within the common library, so they should be within the
Common namespace.

Gets rid of the need to use typename to access the ::type alias.

Allows objects that contain rectangle instances to be constexpr
constructible as well.

const/constexpr variables have internal linkage by default.

This hasn't been used since the project started, so we may as well get
rid of it to keep it from bit rotting.

Prevents potentially using the members uninitialized.

C++17 adds clamp() to the standard library, so we can remove ours in
favor of it.

And other minor style changes

Also minor style changes

Also replaced C headers with the C++ equivalent ones