Add all type declarations

If we want everything exposed from the top-level
module, it needs to be declared here. Ideally,
we'd only need to declare the cython parts but
mypy wants everything.

Please let me know if there's a better way to
do this.

src/flatrtree/__init__.py

@@ -4,7 +4,7 @@
 from flatrtree.rtree import RTree
 from flatrtree.serialization import deserialize, serialize
 
-DEFAULT_DEGREE = 8
+DEFAULT_DEGREE: int = 8
 
 __all__ = [
     "RTree",

src/flatrtree/__init__.pyi

@@ -1,4 +1,32 @@
-from flatrtree.rtree import RTree
+from typing import Callable, Iterator, List, Optional, Tuple
+
+DEFAULT_DEGREE: int
+
+class RTree:
+    def __init__(self, count: int, refs: List[int], boxes: List[float]): ...
+    def search(
+        self, minx: float, miny: float, maxx: float, maxy: float
+    ) -> Iterator[int]: ...
+    def neighbors(
+        self,
+        x: float,
+        y: float,
+        box_dist: Callable[
+            # x, y, minx, miny, maxx, maxy
+            [float, float, float, float, float, float],
+            float,  # dist
+        ],
+        item_dist: Optional[
+            Callable[
+                # x, y, ref
+                [float, float, int],
+                float,  # dist
+            ]
+        ] = None,
+    ) -> Iterator[Tuple[int, float]]: ...
+
+def serialize(rtree: RTree, precision: int) -> bytes: ...
+def deserialize(data: bytes) -> RTree: ...
 
 class HilbertBuilder:
     def add(self, ref: int, minx: float, miny: float, maxx: float, maxy: float): ...