core/gpu: start on usingnamespace issue and new zig split usage

[xdBronch]

beginning work on #796, wasnt sure how to handle callbacks.zig and types.zig since they have so many more decls than the others so just left as is for now.

just merged was slightly new usage of std.mem.split ziglang/zig#15579. The old split function still works but is just an alias for splitSequence and is being deprecated.

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