Stateful Tile Fetching

[c1m50c]

Description

Resolves #13 by enabling stateful tile fetching via splitting out BatchTileFetcher and IndividualTileFetcher into traits. AFAIK, this shouldn't break any existing code, due to the following impl blocks:

impl<F> BatchTileFetcher for F
where
    F: Fn(&[(i32, i32)], u8) -> Result<Vec<(i32, i32, DynamicImage)>, Error>,
{
    fn fetch_tiles(
        &self,
        coordinate_matrix: &[(i32, i32)],
        zoom: u8,
    ) -> Result<Vec<(i32, i32, DynamicImage)>, Error> {
        (self)(coordinate_matrix, zoom)
    }
}

impl<F> IndividualTileFetcher for F
where
    F: Fn(i32, i32, u8) -> Result<DynamicImage, Error>,
{
    fn fetch_tile(&self, x: i32, y: i32, zoom: u8) -> Result<DynamicImage, Error> {
        (self)(x, y, zoom)
    }
}