Add string view support (only impl, no tests) #233

Job	Run time
ubuntu (g++, standard)	1m 13s
windows (Win32, standard)	1m 8s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 13s
windows (Win32, PUGIXML_WCHAR_MODE)	3m 4s
ubuntu (g++, PUGIXML_COMPACT)	1m 16s
windows (Win32, PUGIXML_COMPACT)	1m 5s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 4s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	1m 7s
ubuntu (clang++, standard)	1m 2s
windows (x64, standard)	1m 38s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	57s
windows (x64, PUGIXML_WCHAR_MODE)	1m 32s
ubuntu (clang++, PUGIXML_COMPACT)	1m 1s
windows (x64, PUGIXML_COMPACT)	1m 47s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	55s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 49s
macos (clang++, standard)	46s
macos (clang++, PUGIXML_WCHAR_MODE)	47s
macos (clang++, PUGIXML_COMPACT)	47s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	45s
	24m 56s

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