preserve_source_value_flags
also: - fixed natvis file for v3 namespace - small refactors
also: - made single header generation script `#pragma once`-aware
toml::table
also: - fixed incorrect `noexcept` specifications on many functions - fixed missing `#include <initializer_list>` - removed unnecessary uses of `final`
also: - removed `TOML_LARGE_FILES` - removed unnecessary template machinery (esp. where ostreams were involved) - made all overloaded operators 'hidden friends' - documentation fixes - version bump - this will form the foundation of v3
