Support arbitrary block index functions in blockwise

[tomwhite]

The idea here is to generalize blockwise slightly so you express a dependency of an output array block on input array blocks that you can't express using the index notation.

The example in the test is a simple form of merge_chunks where three input blocks are merged into one output block. (This will be useful in improving reduction, but that is not included in this PR.)

The reason we need this change is so that we can improve fusion optimizations. Currently merge_chunks uses the opaque map_direct function, which i) assumes that the array being merged has been materialized as a Zarr array, and ii) doesn't give any information about block mapping. By re-writing merge_chunks as a blockwise operation we get information about the block mapping that we will be able to use to fuse the merge_chunks with earlier operations in the DAG.