const string for c10::string_view

[cpuhrsch]

This PR typedefs strings that are meant to be constant, i.e. read-only. They can then be optionally replaced by std::string_view.

Also adds the ever so important "#pragma once" to the header files.

This will also need to change the key type of the Vocab hash table, since the string passed to find is now just a string_view, which doesn't match the type.