Split entire core namespace into declaration and definition headers

[AnastaZIuk]

then have a declaration of the functions in core/decl/math/floatutil.h and the definition in core/def/math/floatutil.h like other templates

Originally posted by @devshgraphicsprogramming in https://github.com/Devsh-Graphics-Programming/Nabla/pull/205#discussion_r702157783