Merge branch 'jh/alive_coercions' into 'master'

theories/lifetime/lifetime.v

theories/lifetime/lifetime_sig.v

theories/lifetime/model/borrow.v

theories/lifetime/model/definitions.v

theories/lifetime/model/primitive.v

theories/lifetime/model/reborrow.v

theories/typing/borrow.v

theories/typing/examples/get_x.v

theories/typing/examples/unbox.v

theories/typing/function.v

theories/typing/lft_contexts.v

theories/typing/lib/cell.v

theories/typing/lib/fake_shared_box.v

theories/typing/lib/option.v

theories/typing/lib/refcell/ref.v

theories/typing/lib/refcell/ref_code.v

theories/typing/lib/refcell/refcell.v

theories/typing/lib/refcell/refcell_code.v

theories/typing/lib/refcell/refmut.v

theories/typing/lib/refcell/refmut_code.v