Create sp-weights crate to store weight primitives

[KiChjang]

Part of paritytech/polkadot-sdk#256.

In this PR, we create a new primitives crate called sp-weights to store the Weight struct and its associated traits and types. Some existing types related to dispatch have also been relocated to frame_support::dispatch, as I feel like they had never made sense to be put under the weights umbrella.

Aside from that, the main motivation here is to recognize that the notion of weights reaches far beyond FRAME, and so it should be a crate separate from frame_support or sp-runtime.

polkadot companion: paritytech/polkadot#5987
cumulus companion: paritytech/cumulus#1610

[KiChjang]

I'm attempting to try a novel way to deprecate types that have been moved, and one of the ways that I can think of is to create new subtraits and type alias and immediately deprecating them, telling the FRAME dev that the trait/type has been moved to another location.

Would be nice if I get some eye balls on this to ensure that this is worthwhile to pursue.

[KiChjang]

The idea here is to create code that still works like the old way, but would emit a deprecation warning about the usage. This would make it easier for people to migrate over to the new location by allowing them to do so at a time of their choosing, while still having code that works.

[kianenigma]

looks decent to me, just not sure when and based on what criteria are we going to remove the old code? we also have not removed the old macros now for more than a year and it is unclear to me when we are going to.

If there's no clear answer to this, I think it is also reasonable to just break the code downstream instead of keeping a lot of duplicate code around.

[gilescope]

You could stick a comment stating that the old methods will be removed once we get to date x.

[KiChjang]

bot merge