This is the same as the previous commits, but for non-sum NTs.
This also extracts errors into a separate module, which I had hoped to do in
a separate commit, but it's not worth separating them. My _original_ reason
for doing so was debugging (I'll get into that below), but I had wanted to
trim down `ele.rs` anyway, since that mess is large and a lot to grok.
My debugging was trying to figure out why Rust was failing to derive
`PartialEq` on `NtError` because of `AttrParseError`. As it turns out,
`AttrParseError::InvalidValue` was failing, thus the introduction of the
`PartialEq` trait bound on `AttrParseState::ValueError`. Figuring this out
required implementing `PartialEq` myself without `derive` (well, using LSP,
which did all the work for me).
I'm not sure why this was not failing previously, which is a bit of a
concern, though perhaps in the context of the macro-expanded code, Rust was
able to properly resolve the types.
DEV-7145
Mike Gerwitz