Fix tracestate implementation

[Nevay]

Found some issues while running the TraceState implementation against my old test cases:

• member limit of 32 was only applied on ::parse(), was not applied in ::with()
• ::parse() discarded tracestates with length > 512; W3C spec specifies this as lower limit, not upper limit:

  Vendors SHOULD propagate at least 512 characters of a combined header.

• ::parse() did not allow empty / whitespace-only members, W3C spec:

  Empty and whitespace-only list members are allowed. Vendors MUST accept empty tracestate headers but SHOULD avoid sending them. Empty list members are allowed in tracestate because it is difficult for a vendor to recognize the empty value when multiple tracestate headers are sent.

This PR also changes the behavior for duplicate keys in ::parse(); previously the last value was used, now the first (=newer) value is used; we might consider discarding the tracestate instead. key=value,key2=value2,key=value3 resulted previously in key=value3,key2=value2, now results in key=value,key2=value2.

[codecov]

Codecov Report

Attention: Patch coverage is 98.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.91%. Comparing base (a1489d3) to head (cb4e37f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1267      +/-   ##
============================================
+ Coverage     83.87%   83.91%   +0.03%     
- Complexity     2136     2144       +8     
============================================
  Files           296      296              
  Lines          6104     6125      +21     
============================================
+ Hits           5120     5140      +20     
- Misses          984      985       +1     

Flag	Coverage Δ
8.1	83.91% <98.33%> (+0.03%)	⬆️
8.2	83.91% <98.33%> (+0.03%)	⬆️
8.3	83.91% <98.33%> (+0.03%)	⬆️
8.4	83.91% <98.33%> (+0.03%)	⬆️

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Files	Coverage Δ
src/API/Trace/TraceState.php	98.57% <98.33%> (-1.43%)	⬇️

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