ref: Add CacheEntry type and use it for SymCaches and PpdbCaches

[loewenheim]

This adds a new enum CacheEntry that is intended to eventually replace the current representation of cache entries as (essentially) a pair of (CacheStatus, ByteView) and the myriad different errors. As a first step, we use it in SymCacheActor::fetch and PortablePdbCacheActor::fetch and consequently also in the module_lookup module. The code in the latter becomes slightly simpler, in my opinion, but the big win is that we parse the caches only once and from then on pass around the parsed object instead of using a ByteView that has to be parsed over and over again.

This currently requires a development version of symbolic because some additional traits have to be implemented for PortablePdbCache.

EDIT: The most recent version replaces the custom CacheEntry enum with a Result. The semantics stay exactly the same.

[codecov-commenter]

Codecov Report

Base: 64.87% // Head: 64.54% // Decreases project coverage by -0.32% ⚠️

Coverage data is based on head (3edaf3a) compared to base (bbaec01).
Patch coverage: 63.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #929      +/-   ##
==========================================
- Coverage   64.87%   64.54%   -0.33%     
==========================================
  Files          68       68              
  Lines       11797    12012     +215     
==========================================
+ Hits         7653     7753     +100     
- Misses       4144     4259     +115     

Impacted Files	Coverage Δ
...s/symbolicator-service/src/services/ppdb_caches.rs	65.44% <47.05%> (-18.20%)	⬇️
...symbolicator-service/src/services/symcaches/mod.rs	81.76% <56.84%> (-11.48%)	⬇️
crates/symbolicator-service/src/cache.rs	90.75% <65.90%> (-3.24%)	⬇️
...ervice/src/services/symbolication/module_lookup.rs	97.24% <88.88%> (-0.44%)	⬇️
...ervice/src/services/symbolication/symbolication.rs	79.12% <100.00%> (+0.75%)	⬆️
...olicator-service/src/services/symbolication/mod.rs	50.90% <0.00%> (-7.28%)	⬇️

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