lib: minor fixes

[lucab]

First, a big thanks for maintaining this! 💚

I was skimming through the project and saw a few easy cleanups, which are rolled up in this PR:

• removing a stale dev-dependency
• fixing CommonMark syntax for pulldown migration (see Tracking issue (Rustdoc): hoedown -> pulldown migration rust-lang/rust#44229 (comment))
• performing an overall pass for clippy warnings
• adding a simple runnable floating-point example, and to the crate doc as well

[rubdos]

Cool, thanks! I wondered whether the example on line 32 actually ran as a doctest; it does.

[akubera]

Yeah I never thought to run clippy over this code for some reason...

Thanks a lot! I really appreciate it.

[lucab]

For reference, there is one minor clippy warning inside a macro that I didn't fix.

[rubdos]

Maybe we want clippy in travis?