Rename `core::private::global_data`

[brson]

With upcoming changes to the runtime there is going to be a clear distinction between real 'global' and 'kernel-global'. I am thinking that kernel_data might describe it best.

[catamorphism]

There is no longer a type called global_data, though there are some functions with global_data in the name. Closing; reopen with clarification if necessary.