remove sp-std

[gilescope]

This PR seeks to make things a little more normal for regular rust developers coming to substrate. Some previously private types are now not as the types are now stable.

Fixes #2101

[yjhmelody]

Nice, I could close it #2818

[xlc]

Can we deprecate it before delete it? You might be having fun doing this for polkadot-sdk but I am sure parachain teams are all busy and don't really want to repeat this big chunk of changes in our repo on next upgrade.

[koute]

Can we deprecate it before delete it?

Yes, let's not break our downstream users. We should do this in two steps:

1. Remove every use of sp_std in the codebase, deprecate sp_std and disallow any new uses.
2. Remove sp_std itself after a long-enough deprecation period.

Also, considering the size of this PR and the amount of places it will touch it might be easier to do this in multiple PRs instead of changing everything at once (e.g. pick one subdirectory per PR and only change things there).

[davxy]

As @koute said, I suggest to remove usage of sp-std in the codebase (as you've done) but also to wait for complete removal of it.

I'd follow a procedure similar to what has been done for sp-core-hashing and sp-core-hashing-proc-macro here #2306 I.e. put it in the substrate/deprecated folder and schedule removal after 6 months

According to the guidelines we also need to create an issue (maybe we can just use #2101 and add the T15-deprecation tag)

[gilescope]

Agreed. Once I have defeated the CI for this combined PR I will raise incremental PRs that have subsets of these changes in. I think the deprecation route of 6 months seems a good one.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 3/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5568397