Refactor to cleanup and prepare for directory caching

[nyurik]

• there is no point in returning type and compression with every get_tile -- PMTiles keeps this data in the header, so it is constant, and if the user needs it, they can get it directly. Returns bytes now.
• add Entry::is_leaf helper
• add Header::get_bounds and Header::get_center (tilejson structs)
• no need for AsyncPmTilesReader<B: ...> type - it has to be specified in the impl anyway
• no need for the backend::read_initial_bytes - it is only used once, has default implementation anyway. Inlined.
• inlined read_directory_with_backend - used once and tiny
• split up the find_tile_entry into two functions - I will need this later to add caching -- the root entry is permanently cached as part of the main struct, but the other ones are not, so needs a different code path.
• added cargo test for default features

[nyurik]

it ended up being more complicated because I would have to re-wrap entry as an option