Implement serde derivations also in no_std

[haerdib]

Is there an existing issue?

• I have searched the existing issues

Experiencing problems? Have you tried our Stack Exchange first?

• This is not a support question.

Description of bug

Many core primitives of substrate only implement serde in std mode. The reason being the efficiency, so I was told. However, in a no_std environment, this forces a lot of workarounds if serde is needed. Is there any chance that you would accept a PR introducing an additional compiler feature that allows to derive serialization also in no_std mode?
(such as here: https://github.com/paritytech/parity-common/blob/master/primitive-types/Cargo.toml#L26)

Steps to reproduce

No response

[bkchr]

Yes that would be okay.

[nazar-pc]

Replacing feature = "std" with feature = "serde" should be sufficient, so users can decide whether to enable it or not separately from std.

[haerdib]

"serde" or "serde_full", as it's called in frame-metadata ?

[bkchr]

Please call it serde. For this you only need to make serde an optional dependency. An optional dependency is directly exposed as feature in cargo.