Support conversion between types

[mxgrey]

Rust has excellent support for converting between data types with the Into, From, TryFrom, and TryInto traits. We could leverage these traits to allow users to register conversions between message types. We could support implicit conversion for any output type that supports Into<T> for its target input.

We could also support implicit conversion when TryInto is available, but this might be an opt-in setting that the user toggles at the diagram level or inside the specific node where the implicit conversion should be allowed. Just like implicit deserialization, an implicit TryInto would direct errors towards the on_implicit_error target.

We should also automatically support implicit conversions from smaller primitives to larger primitives where there is no risk of overflow or loss of precision, e.g. i8 -> i16 -> i32 -> i64, or less obviously u8 -> i16, u16 -> i32.