This converts it into a regular constructor parameter. There's no need
to make this a template parameter on the class when it functions
perfectly well as a constructor argument.
This also reduces the amount of code bloat produced by the compiler, as
it doesn't need to generate the same code for multiple different
instantiations of the same class type, but with a different fill value.
Several classes have a lot of non-trivial members within them, or don't
but likely should have the destructor defaulted in the cpp file for
future-proofing/being more friendly to forward declarations.
Leaving the destructor unspecified allows the compiler to inline the
destruction code all over the place, which is generally undesirable from
a code bloat perspective.
