Proto/Michelson: Move the definition of boxed_list to Script_list

Context

The module Script_list defines some helper functions to operate on boxed_lists, but the type itself is defined in Script_typed_ir. It seems the more proper place for it is Script_list though. Additionally, the type has been made private and some helper functions were added where required. Ideally, Script_list.t should be made abstract, but at the moment it's unfortunately impossible, because some instructions in Script_typed_ir.kinstr and Script_typed_it.continuation GADTs contain boxed lists. Thus the boxed list type definition must be known to Script_typed_ir, so that the compiler could assert injectivity.

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