Restructure repo

[andrew-fleming]

Fixes #177.

Draft because docs still need to be updated and tests may be moved to contracts/test/. The latter may be better addressed in another issue/PR

This PR proposes to:

• Move all modules into a single contracts workspace
• Output artifacts as contracts/artifacts (as opposed to contracts/src/artifacts/)
• Remove node_modules from compact module imports
• Improve ts test utils representation: MaybeString -> Maybe<string>
• Clean up compact and build turbo tasks
• Split utils directory into security and utils
  • Security module includes Pausable and Initializable
  • Utils module only has Utils (Might be worth considering a dif name because the path is utils/Utils)

Docs:

• Update imports according to new structure
• Combine Ownable and AccessControl guides into a single Access page (same with API)
• Switch the order of the sample import and module title -> Now the title comes first and the import second
• Add security guide page and API

The downside with this approach is that it's not easy to filter tests and compilations and thus, these actions take a much longer time than before. If this PR is approved, this is something that should be improved to speed up development

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Considerations that are out of scope (will track with issues if this PR is approved):

1. src should be removed from the import path
2. Tests can be moved to its own directory alongside src; however, it's not clear whether this makes more sense than keeping colocated tests. Requires further discussion and planning

[emnul]

Looks a lot cleaner! Longer compilation times shouldn't be too worrisome. It should be very fixable with more work on our compilation tooling at a later date