Implement strstr() and strpbrk() from string.h

Created by: azymohliad

This is my first attempt to contribute to this project and I'm not experienced in Rust, especially with its C FFI, so please review carefully. I'm not sure about some of my changes, I'll ask more specifically in diffs. Sorry for any mistakes in advance.

I have test failure on "math", but if I remove it from tests/Makefile, my test performs correctly.

P.S. If somebody is already doing it - please feel free to close my pull request. I'm just creating it for more convenient feedback.