Implement a safe mapped memory wrapper

[c0gent]

... to handle any host memory mapped to a buffer or image object on the device.

The enqueue_map... functions are currently returning pointers and leaving it completely to the caller. It's also currently possible to map some memory by setting the right flags when creating a buffer or image. Doing this just leaves everything in the hands of the caller (should probably error if either of the mapping CL_MEM_ flags are on to prevent this for now -- or at least provide something else in the way of warnings as it's not marked unsafe to create either mem object).

This new type should automatically track any pending reads and writes as well as ensure that all commands related to it have completed before unmapping and dropping.

[c0gent]

Done: MemMap