accounts/abi: fix event unpack into slice

+ The event slice unpacker doesn't correctly extract element from the
slice. The indexed arguments are not ignored as they should be
(the data offset should not include the indexed arguments).

+ The `Elem()` call in the slice unpack doesn't work.
The Slice related tests fails because of that.

+ the check in the loop are suboptimal and have been extracted
out of the loop.

+ extracted common code from event and method tupleUnpack