[WIP] string_view support

[halx99]

Don't needs merge, just a idea

• All string setter function use string_view_t
• API compatible with 1.11.x, but binary incompatible.
• Add explicit boolean type for store bool value as string value true, false and avoid compiler ambiguous match const char_t* as scalar type 'bool'
• Add length field to support getter string as string_view_t, it's useful for avoid unnecessary strlen operations, could track this branch: https://github.com/halx99/pugixml/tree/dev

[brandl-muc]

I think you would need to use basic_string_view<char_t> instead of string_view.

[halx99]

Yes, will change later

[halx99]

Close, replaced by the super newer: #457