[Work Item]: Rename various feature gates

[BoxyUwU]

We should use a naming scheme for the const args stuff where it's X_generic_const_args where X is what kind of const arg we now support. E.g. type_const_generic_const_args struct_expr_generic_const_args const_item_generic_const_args etc.

We should rename generic_const_args to const_item_generic_const_args. And split min_generic_const_args into type_const_generic_const_args and struct_expr_generic_const_args.

This issue has been assigned to @NthTensor via this comment.

[BoxyUwU]

@rustbot assign @NthTensor

[NthTensor]

https://github.com/NthTensor/rust/tree/informative_generic_const_args_names

@BoxyUwU how does this look? Renamed the generic_const_args feature to const_item_generic_const_args, and renamed uses of gca in the internals to const_item_gca.