Continue parsing on parse error and collect all parse errors

[zkat]

Modern parsers don't just stop at the first error they find--they continue optimistically as best they can, either collecting or printing errors as they go.

kdl-rs should do this, too, by returning a wrapper around a collection of parse errors instead of a single error.

This issue can dovetail with #67, since we're going to be kinda rewriting the parser anyway.