Add strcspn implementation

[umutoztunc]

I thought of implementing it using a 256-bit bitmap. Like storing characters in s2 like a blacklist to stop iteration of s1. However, it might be an overkill to create a bitmap for overall use-cases. Under most circumstances, the length of s2 should be way smaller than s1. Thus, I decided to copy the implementation from Bionic like some other functions in string.h.

I was planning to write some test cases, but I noticed there are no test cases for functions like strchr and the test structure looked a bit weird to me. It might be better to refactor the tests at some point. Still, I can write tests similar to fgets if you'd like.

[Cucuzacu]

Hello! If you are still interested about this project, I decided to make a fork: https://github.com/Cucuzacu/ELVM-Remastered. I will constantly update it to support more and more C functions. I added your strcspn implementation to my fork, along with scanf. Maybe you can help?