Persist symbol index to disk

[jakebailey]

For #433.

Either in the regular database location with some hashing, or in a new cache in the project root.

[MikhailArkhipov]

Looks like we don't need this.

[DonJayamanne]

Either way, please remember, VSC had the ability to provide a caching mechanism (internally VSC uses the fs to persist the data).
Data can be cached per workspace or in a global context.

@MikhailArkhipov @jakebailey /cc

[jakebailey]

Symbol indexing is so incredibly fast that we decided that persistence is just not needed. Persistence for other parts of the language server isn't off the table, just symbols.

[qubitron]

Per Don's comment, see: https://code.visualstudio.com/blogs/2019/02/19/lsif

[jakebailey]

#472 is the issue for persistent analysis (outside of the symbol index), but I did see lsif when it was released and I'm not sure how much of it would be helpful, at least for our situation.

Because it's (sort of) a language server protocol message cache, the moment something changes, we'd incur the cost of reanalyzing everything over again, as the backing data would be different. That format omits completion, but retains hover/references/definition/etc, which at least from our POV are pretty linked to the analysis as a whole.

When it comes to symbols, that's still really fast and likely doesn't need to be persisted.