Add other altrep methods

[EmilHvitfeldt]

As far as I can see, the following can be implemented

• R_set_altreal_Get_region_method()
• R_set_altreal_Is_sorted_method()
• R_set_altreal_Max_method()
• R_set_altreal_Min_method()
• R_set_altreal_No_NA_method()
• R_set_altreal_Sum_method()
• R_set_altrep_Coerce_method() to be done in #47
• R_set_altrep_Duplicate_method()
• R_set_altrep_DuplicateEX_method() no one else has done it https://github.com/search?q=org:cran+R_set_altrep_DuplicateEX_method&type=code
• R_set_altrep_Unserialize_method() only if needed https://svn.r-project.org/R/branches/ALTREP/ALTREP.html#serialization
• R_set_altrep_UnserializeEX_method()
• R_set_altrep_Serialized_state_method() only if needed https://svn.r-project.org/R/branches/ALTREP/ALTREP.html#serialization

Not quite sure that R_set_altreal_Get_region_method() is possible.