Test of new binder API: Find unused definitions

[beta-ziliani]

To be merged on top of red-green-trees.

It adds as a test case in the cargo-sphere with the example from find-unused-definitions, but using the new binder and output from the backend.

Some functions are almost literal translations. For instance, collect_definitions still uses the cursor. Other functions exploit the information from a typed AST. visit_contract is a good example.

It adds a few helper functions:

• Binder::navigate_to to flow from an identifier node id to a definition.
• Definition::node to obtain the inner SyntaxNode.
• SyntaxNode::create_cursor (self-described).
• re-exports the ir2 intermediate language as ast in the backend mod.

NOTE: I screw my commits, so they're unparseable on their own. The best way to review it is to just focus on the test/src/ast_api folder.

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Latest commit: 106cc61

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[ggiraldez]

Migrated to manastech#5 to keep track of the branch, because the dependant red-green-trees is not likely to be merged in the near future.