Adjust the trace structure.

[jmjoy]

To add logging and metrics in the future, adjust the structure of the trace.

1. Rename mod context to trace.
2. Move reporter to trace, and rename Reporter to TraceReporter, and do so for implementers.
3. Add the logging mod and base structure.

[codecov-commenter]

Codecov Report

Merging #40 (092ea33) into master (fa8ba85) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   45.23%   45.23%           
=======================================
  Files          14       14           
  Lines         378      378           
=======================================
  Hits          171      171           
  Misses        207      207           

Impacted Files	Coverage Δ
src/skywalking_proto/v3/mod.rs	0.00% <ø> (ø)
src/trace/propagation/context.rs	100.00% <ø> (ø)
src/trace/propagation/decoder.rs	91.17% <ø> (ø)
src/trace/propagation/encoder.rs	100.00% <ø> (ø)
src/trace/span.rs	0.00% <ø> (ø)
src/trace/system_time.rs	0.00% <ø> (ø)
src/trace/trace_context.rs	24.48% <ø> (ø)
src/trace/tracer.rs	61.79% <ø> (ø)
tests/propagation.rs	100.00% <ø> (ø)
tests/trace_context.rs	0.00% <ø> (ø)

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[wu-sheng]

Move reporter to trace, and rename Reporter to TraceReporter, and do so for implementers.

Mostly, we recommend only having one global reporter for trace, log, and metrics.

[jmjoy]

Move reporter to trace, and rename Reporter to TraceReporter, and do so for implementers.

Mostly, we recommend only having one global reporter for trace, log, and metrics.

Alright.