Add unit (and doc) tests for de_impl_opt.rs

[rnag]

There seems to be some differing assumptions on what should be None and what shouldn't.

I find the examples section hard to read. Rather than having many examples, it may be better to have unit tests - and examples (doc tests) in src/de_impl_opt.rs doc strings.

Somewhere there should be a dense table/section where the behaviour can be seen at a glance - possibly in the doc string itself. I like the # Returns section, explaining behaviour, but this could be complemented with more comprehensive cases for each of the input types str, i64, f64, u64.

Originally posted by @corneliusroemer in #5 (review)