Rework `features` param in `rust_source()`

[Ilia-Kosenkov]

A general improvement of rust_source().

• Require extendr_deps and dependencies to be named lists
• Merge (instead of appending) extendr_deps into dependencies -- if dependencies contains extendr-api entry, it will be overwritten by that of extendr_deps
• Make features a feature flag variable, allowing "ndarray", "num-complex", "serde", "graphics" features (and their combinations)
• Correctly set features property on extendr-api no matter how it is defined
• Add explicit references to relevant packages for all features but graphics

Fixed #239

[CGMossa]

Please rename the example that doesn't contain either, to something without either.