Reimplement #[bitcode(with_serde)]

[caibear]

There have been many requests for supporting 3rd party crates such as: time, rust_decimal, uuid, and chrono. Rather than implementing support manually for these crates and more, an easier solution is to allow types implementing serde::Serialize inside types implementing bitcode::Encode.

This was previously a feature in 0.5, but we didn't reimplement it in the 0.6 rewrite due due to technical limitations. I have since found a path forward and intend to add this feature to 0.6.

The downsides of using serde instead of implementing these types in native bitcode are:

1. Worse performance
2. Requiring #[bitcode(with_serde)] annotations on every field where serde is used
3. Doesn't work well on collection types since it forces everything inside them to use serde

1 and 2 aren't much of an issue if serde is used infrequently. 3 poses more of an issue, but so far none of the types requested are collections. We could mitigate it by implementing some of the popular collections such as IndexMap.

[finnbear]

Downside 4: serialize returns a Result, so encode would either have to return a Result (breaks compatibility with 0.6) or panic on Err from serialize.

Still in favor of this, though!

[caibear]

Implementing this would partially-fully resolve these open issues:
#45 could use serde_repr to serialize enum as u16
#40
#39
#38
#37
#35 bitcode serde already supports recursive types, so you could embed them in bitcode::Encode types
#33
#29 use serde on field you want to skip with serialize impl that does nothing
#27
#24
#13
#58

[vnghia]

I would like to ask if we use #[bitcode(with_serde) for example with uuid::Uuid, do we get to choose which representation we want (something similar with {de}serialize_with) because I think most people will want to encode an Uuid with u128 instead of a str (no one will open the bitcode message to read anyway :D).

[finnbear]

most people will want to encode an Uuid with [bytes] instead of a str

The correct choice is made automatically, considering uuid checks whether the format is human readable. bitcode is not human readable, so uuid uses raw bytes.