Refactored simple keys so that validating doesn't require popping indents, and so popping indents (and adding the end map) is independent of when we validate the simple key

Fixed bug in anchors with no content. This involved refactoring the 'implicit sequence' concept (where a map and a sequence start on the same indent, but we read the sequence as more indented since the '-' is visually an indent).

Converted indexing to std::size_t, and fixed the Node templated overloads to properly index any index type (determining what is an index type is a bit of a hack - it should be is_convertible<T, std::size_t> (I think), but I just explicitly wrote down a list)

Added templated Read() function that creates the output variable itself (so you don't need to have a temp variable)

Added emitting for a YAML::Node (instead of the ad-hoc std::ostream overload) so it'll actually emit valid YAML always