Lifts over a `variant` to another coordinate. Returns a converted and
realigned variant if succeeded to lift over, otherwise nil.
`indexed-chain` supplies conversions between sequences and must be indexed by
`varity.chain/index`. `target-seq-reader` is a reader instance of the target
sequence that the variant coordinate is converted to.

`liftover-variant` without checking reference sequence and realignment.

Lifts over variants data from chains applied chain/index and seq-reader.
Variants that succeed in liftover will be put in :success,
and those that failed will be put in :failure.