Span links stored as Vector instead of EvictedQueue

[cijothomas]

Continuing #1293. This PR modifies Span links - they are now stored as Vec, and the limit enforcement is done without the previous "eviction" behavior. A simpler logic of keeping the 1st "N" links is performed now. Have not done more benchmarks given the perf gains are already proven in prior PRs.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
opentelemetry-datadog/src/exporter/model/mod.rs	85.2% <100.0%> (ø)
opentelemetry-sdk/src/export/trace.rs	40.0% <ø> (ø)
opentelemetry-sdk/src/testing/trace/mod.rs	80.8% <100.0%> (ø)
opentelemetry-sdk/src/trace/links.rs	100.0% <100.0%> (ø)
opentelemetry-sdk/src/trace/mod.rs	100.0% <100.0%> (ø)
opentelemetry-sdk/src/trace/span.rs	95.8% <100.0%> (+0.2%)	⬆️
opentelemetry-sdk/src/trace/span_processor.rs	82.8% <100.0%> (ø)
opentelemetry-sdk/src/trace/tracer.rs	94.6% <100.0%> (+0.2%)	⬆️
opentelemetry-zipkin/src/exporter/model/span.rs	96.1% <100.0%> (ø)
opentelemetry-proto/src/transform/trace.rs	38.8% <50.0%> (ø)
... and 3 more

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[cijothomas]

@jtescher could you re-review and merge if good?

[jtescher]

Looks great thanks @cijothomas