Common parser string buffer

[Chlumsky]

It may be slightly beneficial for performance if the generated parser class had a common string buffer member variable instead of temporaries in individual functions, namely:

json-cpp-gen/src/types/StructureType.cpp

Lines 24 to 25 in bb1a666

	// TODO make key a class member to reduce the number of allocations
	body += indent+generator->stringType()->name().variableDeclaration("key")+";\n";

json-cpp-gen/src/types/EnumType.cpp

Lines 15 to 16 in bb1a666

	// TODO make str a class member to reduce the number of allocations
	body += indent+generator->stringType()->name().variableDeclaration("str")+";\n";

[Chlumsky]

Implemented in 91fb66a