Add no_std support and introduce feature std.

[0ndorio]

This introduces no_std support and a new default feature called std. It also replaces skeptic as documentation testing dependency with the far more light weight doc_comment.

This has currently two known problems:

1. During the doctest build step the following error message appears. It seems like this based on an doctest internal problem and has now further consequences as even after the message is printed all tests run properly (related: No global memory allocator error when testing no_std crate that does link to std rust-lang/rust#54010, liballoc doc tests are "silently" failing rust-lang/rust#52243).

      Doc-tests bendy
   error: no global memory allocator found but one is required; link to std or add #[global_allocator] to a    static item that implements the GlobalAlloc trait.
   
   error: aborting due to previous error
   

2. As soon as doc_comment is used with the #[cfg(test)] attribute it is not working as expected and won't run the doc tests. I assume that the reason here is rustdoc not using the test flag (Revert "Set test flag when rustdoc is running with --test option"  rust-lang/rust#61199). So this will not work until Stabilize cfg(doc) rust-lang/rust#61351 and rustdoc: set cfg(doctest) when collecting doctests rust-lang/rust#62213 are merged and we change it into #[cfg(doctest)].