Make backends store bytes instead of strings

This is, for the majority of backends, the native format anyway, and so
we save encoding and decoding when using the PickleCodec or if (in future)
a orjson Coder was to be added.

For the JsonCodec, the only thing that changed is the location where the
JSON data is encoded to bytes and decoded back again to a string.

fastapi_cache/backends/memcached.py

@@ -9,14 +9,14 @@ class MemcachedBackend(Backend):
     def __init__(self, mcache: Client):
         self.mcache = mcache
 
-    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[str]]:
-        return 3600, await self.mcache.get(key.encode())
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
+        return 3600, await self.get(key)
 
-    async def get(self, key: str) -> Optional[str]:
-        return await self.mcache.get(key, key.encode())
+    async def get(self, key: str) -> Optional[bytes]:
+        return await self.mcache.get(key.encode())
 
-    async def set(self, key: str, value: str, expire: Optional[int] = None) -> None:
-        await self.mcache.set(key.encode(), value.encode(), exptime=expire or 0)
+    async def set(self, key: str, value: bytes, expire: Optional[int] = None) -> None:
+        await self.mcache.set(key.encode(), value, exptime=expire or 0)
 
     async def clear(self, namespace: Optional[str] = None, key: Optional[str] = None) -> int:
         raise NotImplementedError