Implementations for standard library types

[milibopp]

By comparison, serde implements its Serialize trait for a lot more types than abomonation does. It might be a nice idea to gradually add more types from std to avoid ergonomic issues caused by missing implementations.

To get started, I added a trivial implementation for PhantomData in #4, because I think it will help make integrating nalgebra and abomonation in dimforge/nalgebra#277 more ergonomic. Other types will likely be more work ;)

[frankmcsherry]

This is a good issue! I can think of a few things in the standard library that are tempting but haven't been done yet for various reasons. Perhaps we can leave the issue open as a place for people to mention types they would like, or to read about why some things are hard. For example:

1. The Rc type is pretty popular, but if abomonation exposes a &Rc<_> backed by immutable data, then operations like clone() will not actually succeed. They tweak a cell wrapping the reference counts (strong/weak) and believe that this means memory will be kept around even after the source reference is discarded, which it will not.

2. The HashMap type is pretty popular too, but currently its backing memory is a bit of a tangle (last time I checked: a raw pointer with some assumed structure, navigated carefully by the implementors). I don't think there is anything in the HashMap signature that prevents exposing a &HashMap backed by immutable data, I'm just not personally comfortable being the one that walks through the fields and sets things appropriately (e.g. if the key field has type String we would need to visit each key and set their pointer appropriately, but HashMap does not let you directly mutate keys, understandably).