Switch to using libc dependency

[AldaronLau]

There should be no MSRV concerns with this.

Eventually libc 0.2 will stop receiving updates, when libc 1.0 with a new MSRV policy is out, but until then the MSRV is compatible. WhoAmI 1.0 support is only guaranteed until then end of this year, although it will be supported for up to 3 more years if possible.

The benefits to using their definitions should outweigh any other concerns.

[AldaronLau]

From discussion: #91 (comment)