Add some temporary printlns

Because who doesn't love println debugging?

Now, running the (failing) test case added in the previous commit, we'll
see:

```
---- toplevel::test::multi_use_with_layered_dirs stdout ----
parsing file at path='"root.witx"' root='"/"'
canonical-path='"/root.witx"'

found a use statement, parsing file at "subdir/child.witx"
parsing file at path='"subdir/child.witx"' root='"/"'
canonical-path='"/subdir/child.witx"'

found a use statement, parsing file at "sibling.witx"
parsing file at path='"sibling.witx"' root='"/"'
canonical-path='"/sibling.witx"'
thread 'toplevel::test::multi_use_with_layered_dirs' panicked at 'parse: Io("/sibling.witx", Custom { kind: Other, error: "mock fs: file not found" })', src/toplevel.rs:151:10
```

Note that root is always `/` even when we're evaluating the paths found
in a document that lives in a subdirectory! See how the canonical path
in the last block resolves to '/sibling.witx'?

We found you, Mr. Bug 🐛 🔍😃

Author: katelyn martin

tools/witx/src/toplevel.rs

@@ -43,11 +43,14 @@ fn parse_file(
     definitions: &mut Vec<Definition>,
     parsed: &mut HashSet<PathBuf>,
 ) -> Result<(), WitxError> {
+    eprintln!("parsing file at path='{:?}' root='{:?}'", &path, &root);
     let path = io.canonicalize(&root.join(path))?;
+    eprintln!("canonical-path='{:?}'", &path);
     if !parsed.insert(path.clone()) {
         return Ok(());
     }
     let input = io.fgets(&path)?;
+    eprintln!(""); // this one is just for space
 
     let adjust_err = |mut error: wast::Error| {
         error.set_path(&path);
@@ -67,6 +70,7 @@ fn parse_file(
                 definitions.push(def);
             }
             TopLevelSyntax::Use(u) => {
+                eprintln!("found a use statement, parsing file at {:?}", &u);
                 parse_file(u.as_ref(), io, root, validator, definitions, parsed)?;
             }
         }