remove SmallVec and LazyIndexMap from json value

[davidhewitt]

This simplifies the jiter public API (particularly on JsonValue) by simplifying the JsonObject and JsonArray types to just use Arc<Vec<...>> rather than smallvec in the array case and LazyIndexMap in the object case. This hasn't affected the way that we parse or the algorithmic complexity, it's a pure simplification.

At the same time we completely remove LazyIndexMap which attempted to be a thread-safe data structure which lazily constructed a mapping for iteration to guarantee uniqueness. I'm reasonably sure it became a "jack-of all trades, master of none" situation and there are bugs in it, maybe also performance costs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.36%. Comparing base (4cefa8b) to head (a3c9ea3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   88.71%   88.36%   -0.36%     
==========================================
  Files          13       12       -1     
  Lines        2207     2106     -101     
  Branches     2207     2106     -101     
==========================================
- Hits         1958     1861      -97     
+ Misses        153      151       -2     
+ Partials       96       94       -2     

Files with missing lines	Coverage Δ
crates/jiter/src/lib.rs	93.33% <ø> (ø)
crates/jiter/src/value.rs	81.02% <100.00%> (-0.09%)	⬇️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4cefa8b...a3c9ea3. Read the comment docs.

[codspeed-hq]

CodSpeed Performance Report

Merging #184 will improve performances by 12.66%

_{Comparing dh/simpler-value (a3c9ea3) with main (9983083)}

Summary

⚡ 2 improvements
✅ 68 untouched benchmarks
⁉️ 3 dropped benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⁉️	lazy_map_lookup_1_10	9.2 µs	N/A	N/A
⁉️	lazy_map_lookup_2_20	25.8 µs	N/A	N/A
⁉️	lazy_map_lookup_3_50	52.6 µs	N/A	N/A
⚡	medium_response_jiter_value	41.3 µs	37.1 µs	+11.38%
⚡	sentence_jiter_value	9 µs	8 µs	+12.66%