Fix path of imports for Rust 2018

[qinsoon]

In GitLab by @caizixian on Dec 3, 2018, 09:57

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the
2018 edition!
    = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>

rust-lang/rust#53130

[qinsoon]

This is done in #67.