core: Add connection graph and incremental reference resolution

[artempyanykh]

This PR on a high-level does 3 things:

1. In addition to Cst, it introduces Ast and Sym; level of abstraction goes as Cst -> Ast -> Sym; each level having less superfluous details. Right now mostly Cst and Sym are used, with Ast akwardly sitting in the middle, but future work may uncover more use for Ast.
2. Introduce Conn - a connection graph between document symbols that can be incrementally updated whenever symbols change. The crucial parts are incremental updates and that when symbols are not changing (e.g. when just typing text rather than adding links/sections) there's no need to recompute the graph.
3. Refactor the Refs and Diag to use the connection graph rather than doing from scratch recomputing on every keystroke.

TODO:

• Add a flag to toggle incremental updating of the Conn graph.
• Add a flag to toggle paranoid mode instead of a cmdline option.
• Fix tests on windows (paths strike again 💀)
• Add tests for tags

In later PRs:

1. Use Conn graph to more granularly identify the "blast radius" of an edit for diagnostic recalculation (instead of recomputing for all docs in the folder).
2. MORE REFACTORING!1!
3. More explicit dependency tracking between symbols in the Conn graph.