Use source generator for type mapping

[alexeyzimarev]

Right now, mapping event types to classes is done using a dictionary. It requires either to register types explicitly or use assembly scanning and attributes. Assembly scanning works with EventType attribute. Dictionaries are relatively fast, but still slower than direct calls. For example, generic-based type map used to map commands to command handlers is by order of magnitude faster than the event type map.

As most of us use the attribute anyway, it should be possible to generate code for event type mapping, both forwards and backwards (type->name and name->type).

The first step could be eliminating assembly scanning, so Eventuous gets a bit closer to be AOT-compliant. The first iteration could generate code to add types and names to the existing TypeMap without assembly scanning at runtime.

The second step would be to generate a static function with a switch expression to replace the type map. However, it will kill the option to provide custom type maps and having multiple type maps in one application. However, I don't know how many people use that option and if it's useful at all.
Feedback about using multiple type maps within one app would be great.

[linear]

EVE-53 Use source generator for type mapping