improve handling of non-numeric values in isNumber<T> #134

Job	Run time
build (ubuntu-latest, gcc, 10, Release, fasthash)	2m 32s
build (ubuntu-latest, gcc, 11, Release, xxhash)	2m 30s
build (ubuntu-latest, gcc, 12, Release, fasthash)	2m 55s
build (ubuntu-latest, gcc, 10, Debug, xxhash, -O0 -fno-inline --coverage, true)	2m 57s
build (ubuntu-latest, gcc, 10, Debug, xxhash, true, -fsanitize=address -fsanitize=leak -fsanitize...	2m 1s
build (ubuntu-20.04, clang, 10, Release, xxhash)	2m 23s
build (ubuntu-latest, clang, 11, Release, fasthash)	4m 8s
build (ubuntu-latest, clang, 12, Release, xxhash)	4m 12s
build (ubuntu-latest, clang, 13, Release, fasthash)	2m 32s
build (ubuntu-latest, clang, 14, Release, xxhash)	2m 20s
build (windows-2019, msvc, Release, xxhash)	3m 45s
build (windows-latest, msvc, Release, xxhash)	3m 46s
build (macos-latest, clang, Release, xxhash)	1m 22s
	37m 23s

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