`no_std` support

[jlxip]

This is an issue to propose the idea of considering adding no_std support. include_str! and include_bytes! both have it, so it would make sense for this too.

[gwen-lg]

I have take a look, and it's seem not possible. include_dir use the Path struct and fs module witch can't be avoided.

include_str! seems to use compiler built-in, so it's seems not possible to copy it in the crate.