Refactor validators

[sobolevn]

Right now validators are very complicated.
They do a lot of stuff:

• Type inference
• Conditional logic
• etc

Most of the problematic validators return Tuple or Context types.

Validators must do just one thing: validation.
Other steps should be executed on a higher level.

.instance call steps should be:

1. We infer everything we need: runtime_type, instance_type, etc
2. We register things we need with mro.metadata
3. Then we validate everything we need
4. Error handling / etc