update doc examples to use `once_cell` instead of `lazy_static`

[vidhanio]

this change is useful because of the proposed addition of the once_cell api into std. users using the once_cell pattern will find it easier to swap to the std versions when/if they are merged.

[vidhanio]

also, if wanted i could update bench/src/bench.rs to use once_cell instead of lazy_static as well.

[BurntSushi]

I think I'd prefer to avoid this. I'd rather just wait until the std APIs are available to avoid churn. There's nothing egregiously wrong with lazy_static.