Make generics for pallet parts omissible in construct_runtime

[KiChjang]

Fixes #8743.

This PR makes it possible to omit the generics while declaring the Event, Config and Origin parts in the construct_runtime! macro.

Things left to do:

• Figure out how to make the pallet declarative macros support this
• Implement the feature on the Origin pallet part
• Implement the feature on the Config pallet part
• Issue deprecation notice on the generics for the 3 pallet parts when they are used in construct_runtime!

[KiChjang]

Getting this to work with the pallet declarative macros is really tough, I've been trying different methods for a while now and it is looking like that there's no possible solution for this.

The primary problem that I'm facing now is that decl_event! contains no information about the Pallet/Module struct, so it can't implement SubstratePalletEvent on it since it lacks the trait bounds and generics on the Pallet/Module, which will be used in the impl syntax. I am also unable to move the impl into decl_module! either, since it doesn't contain any information about the Event type.

I have half a mind to simply restrict this feature to pallet proc macros, but then I'll have to figure out a way to feature gate this so that it only works with proc macros, which should be doable but not ideal...

[shawntabrizi]

I have half a mind to simply restrict this feature to pallet proc macros, but then I'll have to figure out a way to feature gate this so that it only works with proc macros, which should be doable but not ideal...

I think this is sensible... also we must be very close to having no decl macros anymore. Maybe working to get that past the finish line is the way to go.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[gui1117]

I don't understand why do we need this after #9681 ?

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.