add some support for std::string_view #185

Job	Run time
windows (Win32, standard)	2m 3s
windows (Win32, PUGIXML_WCHAR_MODE)	2m 41s
windows (Win32, PUGIXML_COMPACT)	2m 24s
ubuntu (g++, standard)	7s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	2m 2s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1s
windows (x64, standard)	2m 22s
ubuntu (g++, PUGIXML_COMPACT)	5s
windows (x64, PUGIXML_WCHAR_MODE)	2m 45s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	6s
windows (x64, PUGIXML_COMPACT)	3m 7s
ubuntu (clang++, standard)	11s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 56s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	4s
ubuntu (clang++, PUGIXML_COMPACT)	1s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	5s
macos (clang++, standard)	4s
macos (clang++, PUGIXML_WCHAR_MODE)	21s
macos (clang++, PUGIXML_COMPACT)	22s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	3s
	20m 50s

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