Support for std::string_view in proxy_base

[misyltoad]

Allows code like the following to work:

std::string_view pretty_name = my_table["pretty_name"];

Otherwise it fails with:

error: conversion from ‘sol::table_proxy<sol::basic_table_core<false, sol::basic_reference<false> >&, std::tuple<const char (&)[12]> >’ to non-scalar type ‘std::string_view’ {aka ‘std::basic_string_view<char>’} requested
 2131 |                         std::string_view psvPrettyName = tTable["pretty_name"];
      |                                  

[Spacechild1]

On the one hand, I can see how this can be useful to avoid unnecessary string copies. On the other hand, I think it can be pretty dangerous:

Because Lua has garbage collection, there is no guarantee that the pointer returned by lua_tolstring will be valid after the corresponding Lua value is removed from the stack.

https://www.lua.org/manual/5.3/manual.html#lua_tolstring

This would at least require a big fat warning in the documentation.