feat(tree_index, #157): implement TreeIndex::peek_entry

[qthree]

Naive implementation of #157
Tests are passing, but idk about performance impact and lifetime rules in LeafNode::search and InternalNode::search - is it even allowed to pass key reference to outside?

[wvwwvwwv]

The compiler is very good at dead code elimination, so there should be no problem in terms of performance (all are inlined). Just briefly looked into it, this seems OK, but I’ll do a thorough code review on the weekend.

[wvwwvwwv]

@qthree can you possibly run cargo bench with/without the change? + mark this PR as ready when you think it's ready.

[qthree]

main:

TreeIndex: peek         time:   [55.716 ns 56.343 ns 56.837 ns]
                        change: [-3.6546% -0.7751% +2.3509%] (p = 0.62 > 0.05)
                        No change in performance detected.

tree-peek-with:

TreeIndex: peek         time:   [61.050 ns 61.810 ns 62.458 ns]
                        change: [+6.8138% +10.045% +13.459%] (p = 0.00 < 0.05)
                        Performance has regressed.

[wvwwvwwv]

I tested similar code in godbolt, but the compiler was able to eliminate the additional key field if inlined. I guess the code is relatively big, so the whole call stack cannot fit into a single compilation unit. I think, you can copy-and-paste the search methods only for peek_entry (search_entry); though most code will be duplicated… performance vs code quality -> performance in this case.