[ide/cache] signature-based module invalidation

Creating this issue to write down an idea for possible significant performance improvements for the cache server (and as a consequence - IDE performance).

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The problem:

Currently if module A uses module B and we change module B in *any* way - module A will invalidate, causing its dependants to invalidate as well and so on. As a result there is a lot of potentially redundant re-typing happening on every change, which especially hurts IDE performance a lot.

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The idea:

We can NOT invalidate module A if module B has changed but it's _signature_ stayed the same, where _signature_ is: defined public types, field names and types.

So if you change something at expression level and that doesn't affect the API of the module - we don't invalidate dependants.

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Notes:

There are potential problems with inlining, but we can be conservative for a start and use the "full" dependency if we inline something from a module and figure out something better later.

Detecting the signature changes might require re-typing the changed module (especially if monomorphs are involved in field types), but it's what we already do anyway and it's still better to re-type one module, detect that its signature hasn't changed and not invalidate its dependants than to always invalidate and re-type like we do now.

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Some extra thoughts by @Simn:

>11:50
I think as a first step we have to model dependents as (module * whatever)
11:51
There's probably three levels: 1. nothing changed 2. signature changed 3. everything changed