feat: speed up constructing Variant objects (#256)

src/dbus_fast/signature.pxd

@@ -18,6 +18,9 @@ cdef class SignatureTree:
 
 cdef class Variant:
 
-    cdef public object type
-    cdef public object signature
+    cdef public SignatureType type
+    cdef public str signature
     cdef public object value
+
+    @cython.locals(signature_tree=SignatureTree)
+    cdef _init_variant(self, object signature, object value, bint verify)

src/dbus_fast/signature.py

@@ -400,6 +400,14 @@ class Variant:
         verify: bool = True,
     ) -> None:
         """Init a new Variant."""
+        self._init_variant(signature, value, verify)
+
+    def _init_variant(
+        self,
+        signature: Union[str, SignatureTree, SignatureType],
+        value: Any,
+        verify: bool,
+    ) -> None:
         if type(signature) is SignatureTree:
             signature_tree = signature
             self.signature = signature_tree.signature