Initial cargo-c conversion

[lu-zero]

• Remove the now-unneeded entries in Cargo.toml
• Update the documentation
• Port the tests to use cargo ctest

Addresses #11

[djc]

Would be nice to be a bit more explicit in your PR about the benefits: what does cargo-c provide over the previous setup?

[lu-zero]

The top 4 are:

• Correct shared library at least for *BSD, Linux, Windows and Mac platforms
• Pkgconfig files containing the correct link line for static linking.
• The ability to merge this crate back to the main rustls and use the normal crate to install the C-API without having to have Cargo.toml options that impact the normal crate usage, see how we do in rav1e and other projects.
• C-API tests integrated with the normal test harness using cargo ctest (yet to be done in this PR)

[lu-zero]

I can complete the set if there is consensus on using it.

[jsha]

I don't plan to integrate cargo-c right now, but thank you for showing how it could work.

[lu-zero]

Feel free to poke me when/if you'd like to add it and I'll freshen the patch. I can spin off the pkg-config generation code so you can use it for #220 in case it could help you.