Improve performance of span context propagation

[rnburn]

Replaces the stringstream-based hex-integer conversion code with faster alternatives. Here are benchmarks from before and after showing significant performance improvements for multikey injection and extraction.

before:

-----------------------------------------------------------------------------
Benchmark                                      Time           CPU Iterations
-----------------------------------------------------------------------------
BM_SpanContextMultikeyInjection             1165 ns       1165 ns     601641
BM_SpanContextMultikeyExtraction            1056 ns       1056 ns     657310

after:

-----------------------------------------------------------------------------
Benchmark                                      Time           CPU Iterations
-----------------------------------------------------------------------------
BM_SpanContextMultikeyInjection               76 ns         76 ns    9649126
BM_SpanContextMultikeyExtraction             288 ns        288 ns    2431295

[jmacd]

Nice!