[WIP] The implements of some v2 concepts for discuss only

[halx99]

CHANGES

• Add length field and update length field at parse & setter time
• All string getter/setter function use string_view_t
• 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'
• Improve performance string literal value assignment with shallow copy support
• Since getter function return string_view, if you needs get char_t* as 1.11.x, just invoke string_view::data(), it never return nullptr, this behavior same with v1.11.x

TODO:

• Since setter is string_view, some tests can modify to use string_view literal svavoid strlen

sub pull-requests

• [WIP] string_view support #441
• [WIP] Improve performance string literal value assignment with shallow copy support #442