[performance] overhaul struct (+ result) caching library for simplicity, performance and multiple-result lookups

[NyaaaWhatsUpDoc]

Description

• updates gotosocial to use codeberg.org/gruf/go-structr instead of codeberg.org/gruf/go-cache/v3/result for result / struct caching which includes the following changes:
  • rewrite caching library for reduced complexity
  • improved caching library code commenting, design
  • now uses just a doubly linked list for tracking entries, instead a linked list + hashmap unnecessarily
  • uses linked lists is more places where advantageous over slices
  • increases the number of internal types that can be pooled / reused
  • adds much more robust tests
  • adds support for loading + caching multiple values at a time
• updates gotosocial to use the new multi value cache loader functions to reduce database calls
• removes the unnecessary step of making the sub-caches package private and only accessible by helper methods
• updates regular cache sweep to keep caches within "60" threshold instead of "80" to give us more headroom during load spikes

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

this diff is a lot more intimidating than it looks in terms of actual gotosocial code, because it includes dropping the old result library from vendor/, and adding the new one.

the separate library itself can be found at https://codeberg.org/gruf/go-structr

[NyaaaWhatsUpDoc]

the cache size estimator i'm going to do in a separate PR, this PR is already large enough, and enough of the internals are similar that the size estimations won't actually be that far off

[tsmethurst]

Good job kimbe 👍