Flatten: keep references to the AST throughout

[CohenArthur]

The Fir will now be able to access AST nodes even deep in the pipeline, and to do so cheaply by just accessing a reference.

In the immediate future, this is useful for typechecking constants and accessing builtin types.

• fir: Remove Default bounds where possible
• flatten: Keep AST information in FlattenData
• flatten: Add new AstInfo enum
• ast-sanitizer: Use new Flatten API
• include_code: Use new Flatten API
• interpreter: Use new Flatten API
• error: Add new with_opt_loc function, depreciate it
• flatten: Add note about symbols from Constants
• name_resolve: Use new Flatten API
• typecheck: Use new Flatten API
• xparser: Add new ast!() macro
• name_resolve: Use xparser::ast! macro
• flatten: Fix AstInfo::symbol in some cases
• rec_typecheck: Use new Flatten API