Reorganize crate exports

[soareschen]

This PR re-organize the exports of various CGP crates, to make them more resistant to breaking changes and allows them to be used in no-alloc no-std environments.

• Move generic error providers to the cgp-error-extra crate.
• Add an alloc feature to cgp-error-extra to enable use of alloc in providers.
• Make private the sub-modules inside CGP crates.
• Explicitly export module items instead of using *.