Switch to LLVM-based code coverage

[ecton]

The code coverage from -Zprofile isn't very good -- it tracks documentation lines as code and has issues within the ecosystem requiring a lot of extra compiler flags. I stumbled upon -Zinstrument-coverage which uses the LLVM coverage tools and should provide much more accurate results.

This PR works and enables it, but unfortunately, it only tracks a single test's coverage due to rust-lang/rust#79899. So, the main branch will use the original coverage until that is resolved -- sounds like it could be a decent task to get involved in those projects.

[codecov]

Codecov Report

Merging #1 (589a019) into main (a2a0e32) will increase coverage by 16.60%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main       #1       +/-   ##
===========================================
+ Coverage   38.33%   54.93%   +16.60%     
===========================================
  Files          16       15        -1     
  Lines         647     1347      +700     
  Branches      294        0      -294     
===========================================
+ Hits          248      740      +492     
- Misses        104      607      +503     
+ Partials      295        0      -295     

Impacted Files	Coverage Δ
local/src/error.rs	4.51% <0.00%> (-10.88%)	⬇️
core/src/schema/view.rs	0.00% <0.00%> (ø)
pliantdb/src/lib.rs
core/src/schema/collection.rs	40.00% <0.00%> (+3.63%)	⬆️
core/src/transaction.rs	48.26% <0.00%> (+7.52%)	⬆️
core/src/test_util.rs	66.66% <0.00%> (+14.03%)	⬆️
core/src/document/revision.rs	100.00% <0.00%> (+21.05%)	⬆️
core/src/schema/view/map.rs	86.36% <0.00%> (+35.69%)	⬆️
core/src/schema/database.rs	100.00% <0.00%> (+35.71%)	⬆️
local/src/open_trees.rs	100.00% <0.00%> (+40.00%)	⬆️
... and 5 more

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