Store keys in ordinary indexes unsliced

[jeltsch]

We shall change the representation of IndexCompact to use Unsliced SerialisedKey instead of SerialisedKey. This will give us the following advantages:

• It is statically guaranteed that there are no additional prefixes and suffixes stored alongside key payloads.

• The overhead for storing slice information, which also includes the overhead from the extra indirection in paths to byte arrays with payloads, doesn’t occur anymore.

[jorisdral]

Note that we currently use Unsliced only in the tie-breaker map of the compact index. The tie-breaker map is intended to rarely be consulted, so some of the operations like <= on Unsliced SerialisedKey are not super optimised. It will convert Unsliced SerialisedKey to SerialisedKey before using <= on that. For the ordinary index, they probably should be optimised by skipping the conversion step.

[jeltsch]

This issue has been resolved by #659, and the optimization request in the above comment has been resolved by #658.