Return Option rather than fail on empty vectors.

[SimonSapin]

#11129 changed the shift, pop and last methods of vectors to return an Option instead of failing on empty vectors. These methods should probably get the same change, to be consistent

• shift_ref
• pop_ref
• mut_last
• mut_shift_ref
• mut_pop_ref

[nathanielherman]

I'll try to fix this!