Reorganize crates in the repository

[jpraynaud]

Why

We have created many crates lately and we feel that we need to re-organize the folder structure to make it clearer.

What

Define a clear separation between "released" crates and "internal" crates, by moving all utility/test crates in a specific folder (e.g mithril-internal).

How

• Move utility crates in a new folder (internal?) of the repository (mithril-persistence, mithril-doc, mithril-doc-derive, mithril-build-script)
• Migrate mithril-common build script in mithril-build-script
• Fix the crates table generation in the release notes to support "released" crates only
• Publish mithril-build-script in crates.io
• The API reference page on the documentation needs to be updated with new crates, and mention an access to crates.io and npmjs.com when appropriate.