Suppressing -Wsign-conversion warnings #143

Job	Run time
posix (TOOLSET=gcc COMPILER=g++ CXXSTD=11 Job 0, boost, ubuntu-20.04, ubuntu:16.04, g++, gcc, g++...	2m 24s
posix (TOOLSET=gcc COMPILER=g++-5 CXXSTD=11,14,1z Job 1, boost, g++-5, ubuntu-20.04, ubuntu:16.04...	3m 26s
posix (TOOLSET=gcc COMPILER=g++-6 CXXSTD=11,14,1z Job 2, boost, g++-6, ubuntu-20.04, ubuntu:16.04...	3m 54s
posix (TOOLSET=gcc COMPILER=g++-7 CXXSTD=11,14,17 Job 3, boost, g++-7, ubuntu-20.04, ubuntu:16.04...	3m 15s
posix (TOOLSET=clang COMPILER=clang++ CXXSTD=11 Job 4, boost, ubuntu-18.04, clang++, clang, clang...	1s
posix (TOOLSET=clang COMPILER=clang++-4.0 CXXSTD=11, Job 5, boost, clang-4.0 libstdc++-6-dev, ubu...	3m 8s
posix (TOOLSET=clang COMPILER=clang++-5.0 CXXSTD=11, Job 6, boost, clang-5.0 libstdc++-7-dev, ubu...	3m 22s
	19m 30s

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