Cached dependencies

[florimondmanca]

Example:

from bocadillo import App
from asyncdeps import depends, cached
from databases import Database

app = App()

@cached
async def get_db() -> Database:
    async with Database("sqlite://:memory:") as db:
        yield db

@app.route("/")
async def home(req, res, message: Database = depends(get_db)):
    res.text = message