Specify classes as fully-qualified names in stubs

[joehoyle]

When stubs are generated, the type annotations don't use a loading \, which means they are interpreted as relative to the current namespace. That's wrong, as all types are relative to the root namespace.

[striezel]

The rustfmt check is complaining about those changes:

/home/runner/.cargo/bin/cargo fmt --all -- --check
Diff in /home/runner/work/ext-php-rs/ext-php-rs/src/describe/stub.rs at line 166:
                 DataType::Object(Some(ty)) => {
                     fqdn.push_str(ty);
                     fqdn.as_str()
-                },
+                }
                 DataType::Object(None) => "object",
                 DataType::Resource => "resource",
                 DataType::Reference => "reference",
Error: The process '/home/runner/.cargo/bin/cargo' failed with exit code 1