feat: router re-write

[andogq]

Complete overhaul of the router, and the handler macro. Instead of the handler macro replacing the function definition with a custom struct which implements a trait in order to 'smuggle' reflection information (function and parameter names) to the runtime, this implementation leaves the function definition as-is and uses linkme to save the information during the linking step. This provides a number of advantages:

• Less codegen in the macro, so faster and less fragile
• Handlers can now be called from within Rust code as standard methods (eg for unit testing)
• No more weird type-erasure hacks

Another significant change is that the macro no longer tries to statically analyse the handler signature (such as checking if the return type is impl ____ to guess if it's a stream). Now, the trait system is used to correctly infer the return type every time! Along with being more predictable, this allows for other non-stream traits to be returned from handlers, such as impl Iterator!