Remove lazy_static dependency

[briansmith]

Originally we used lazy_static in ring::rand because static didn't work for types that implement Drop, as File does. That restriction was removed in rust-lang/rust#44456. I believe it should now be possible to get the same effects using just std::sync::Once.

I see there are some (potential) problems with lazy_static! such as rust-lang-nursery/lazy-static.rs#116 and rust-lang-nursery/lazy-static.rs#117 and rust-lang-nursery/lazy-static.rs#125.

[briansmith]

@bpowers nicely wrote a PR to do this. That PR taught me a lot about this issue; in particular it taught me that switching to direct of of static mut is a bad idea. I had originally overlooked that using static mut directly would require us to use unsafe where we previously didn't need to.

Regarding the issues with lazy_static linked to above, after I read @bpowers's PR I went back through them again and noticed that the discussion of those issues was pointing out how dangerous direct use of static mut is. Also, the issues with lazy_static have been resolved as of lazy_static 1.2. I upgraded ring's dependency to use version 1.2.

There are simpler and marginally better alternatives to lazy_static, e.g. ones that aren't as obfuscated with macros. Also, there have been proposals to provide a similar mechanism in libstd/libcore. Still, right now lazy_static appears to be, at worst, good enough. So I'm closing this issue.