Result<T, E> + return inlined NDT for built-in impls

[oscartbeaumont]

Closes #415

Changes:

• Add impl Type for Result<T, E> implementation
• Regig FunctionResult trait and drop FunctionReturnType enum
• Replace all built-in implementation on non-primitives to return an inline NDT (this is gonna be powerful in userspace)
• impl Type for str to reduce needs for special combinations (and Serde supports it)
• Rename Primtivies::String to Primitives::str to be more consistent now that we have an impl for it.
• #[cfg(feature = "derive")] is no longer required for 3rd party crate features. This should should also save some compile time.