sp-api: Make the generated code act based on `std` in `sp-api` (#14267)

* sp-api: Make the generated code act based on `std` in `sp-api`

Instead of letting the macro generate code that checks if the `std` feature is enabled, it will now
generate code that checks if the `std` feature is enabled for the `sp-api` crate. The old
implementation basically required that the crate in which the macro was used, had a `std` feature.
Now we don't have this requirement anymore and act accordingly the feature in `sp-api` directly.

* Missing feature!

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Co-authored-by: parity-processbot <>