Remove usage of `sp-std` from Substrate (#7043)

# Description

This PR removes usage of deprecated `sp-std` from Substrate. (following
PR of #5010)

## Integration

This PR doesn't remove re-exported `sp_std` from any crates yet, so
downstream projects using re-exported `sp_std` will not be affected.

## Review Notes

The existing code using `sp-std` is refactored to use `alloc` and `core`
directly. The key-value maps are instantiated from a vector of tuples
directly instead of using `sp_std::map!` macro.

`sp_std::Writer` is a helper type to use `Vec<u8>` with
`core::fmt::Write` trait. This PR copied it into `sp-runtime`, because
all crates using `sp_std::Writer` (including `sp-runtime` itself,
`frame-support`, etc.) depend on `sp-runtime`.

If this PR is merged, I would write following PRs to remove remaining
usage of `sp-std` from `bridges` and `cumulus`.

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Co-authored-by: command-bot <>
Co-authored-by: Guillaume Thiolliere <guillaume.thiolliere@parity.io>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: Bastian Köcher <git@kchr.de>