Perf: Optimize the implementation of IndexIter

[KKould]

Feature Request

Currently, the main performance issue: #162 (comment) in the benchmark is the execution of IndexIter

The current implementation of IndexIter is relatively redundant and cumbersome. Its main principle is to use three iterators in order: expression -> index value -> tuple (the primary key is expression -> tuple)
, the current implementation is relatively poor in readability, is there a better implementation?

https://github.com/KipData/FnckSQL/blob/173c39571f560f4b733c8b0608755653ec9dcf50/src/storage/mod.rs#L146-L356

[KKould]

The previous implementation was temporary. Since Index is now almost complete, I will implement iterators for each index to avoid prediction difficulties caused by a large number of branches.