Deprecate raw RuntimeDbWeight::reads_writes calls

[ggwpez]

We often use these functions in migrations to avoid writing benchmarks. This worked mostly fine with 1D weights but does not work for chromatic weights.

The public reads_writes, reads, writes functions should be deprecated and the weight templates need to find another way of accessing these constants.

[ybensacq]

@ggwpez the purpose of this task is just to deprecate the 3 functions ?

[ggwpez]

Not quite... we also need to come up with an alternative for weight templates since they have a legitimate use-case to access these constants. I dont have a good idea, but maybe you can start by just removing this thing?

polkadot-sdk/substrate/primitives/weights/src/lib.rs

Line 75 in 552be48

pub struct OldWeight(pub u64);

If the contracts pallet depends on that then we copy it into it, but otherwise i hope we dont use it anywhere.

[kianenigma]

#3640 will partially address this.