xso: reject attempts to match the same XML attribute in different fields

This was a bit tricky to build, because it is possible to have an
indirection through a `static` there. Thanks to Rust's extensive
const-fn capabilities, though, it's in fact possible to cover all cases.

We still do two different checks to improve user experience. If we can,
from within the proc macro, determine that two fields refer to the same
XML attribute (because their namespace/name values use the same Rust
tokens), then we reject the fields with a clear error message pointing
at both fields.

In the other case, when there's e.g. `#[xml(lang)]` and
`#[xml(attribute(namespace = rxml::XMLNS_XML, name = "lang"))]`, the
macro cannot be sure that XMLNS_XML is in fact the XML namespace. For
that case, we generate code which is evaluated at compile time (and
has no runtime impact) which panics if the namespace and name of two
attribute-matching fields is the same.

The error message will be less clear (because it contains extra,
unchangeable wording like "evaluation of constant value failed" and "the
evaluated program panicked at", which may be a bit confusing) than the
message generated by the macros themselves, but it's a price we have to
pay unfortunately.

Note that this check may seem cosmetic and purely for better user
experience, but it is in fact needed to avoid generating not-well-formed
and/or not-namespace-well-formed XML: As `AsXml` generates `xso::Item`,
where each attribute is emitted separated (and not aggregated in a
map structure), a naive (and efficient) implementation of a writer might
not double-check that no duplicate attributes are generated.

xso-proc/src/error_message.rs

@@ -96,6 +96,24 @@ impl ParentRef {
     }
 }
 
+/// Wrapper around `Path` with a more human-readable, collapsed version of
+/// `Path`.
+pub(crate) struct PrettyPath<'x>(pub &'x Path);
+
+impl fmt::Display for PrettyPath<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut first = self.0.leading_colon.is_none();
+        for segment in self.0.segments.iter() {
+            if !first {
+                f.write_str("::")?;
+            }
+            write!(f, "{}", segment.ident)?;
+            first = false;
+        }
+        Ok(())
+    }
+}
+
 /// Ephemeral struct to create a nice human-readable representation of
 /// [`syn::Member`].
 ///