Consider switching to cdylib

[pravic]

rust-lang/rust#33553

This eliminates .def file usage, enables LTO. But requires 2016-05-21 or later nightly rustc.

Update: it is available since Rust 1.11.

[pravic]

Done. By it still exports some items like "memcmp", "__mulodi4" and others, so .def file is better.