Figure Out XCM Weight Story

[shawntabrizi]

In order for XCM to not depend on any FRAME crates, it defines and uses its own local Weight definition.

For the migration to Weight v1.5 and eventually v2, this probably wont work.

So we have some low hanging fruit now that we need to fix up and make XCM use REAL weight types.

You can see how XCM was migrated for the time being here: #5943

[KiChjang]

I think this is done -- we've moved the Weight definition to its own sp-weights crate in Substrate, and we can now pull that dependency in. This is good as it doesn't depend on sp-runtime or sp-core and is pretty light-weight.