Fixed List type

[ncave]

@alfonsogarciacaro

• I fixed the List type with real tail (as opposed to copy tail), it works now. You can merge it in the optimized-list branch.
• Unfortunately there is no perf improvement vs the copy-tail version (still about 17% slower than the original), so the bottle-neck must be somewhere else. But it should in theory help with the memory usage.
• There is still room for improvement, like making the Head/Tail/Length/IsEmpty/Item iterative instead of recursive, and other optimization attempts that can continue in [WIP] Optimized List module #2124.

[gitpod-io]