fix: make getters to element public

[seemenkina]

Access to intermediate nodes in the tree is required during tree handling . It is not possible to do it directly because the cache is completely private.

Making this function public will solve this problem

fyi @rymnc

[rymnc]

LGTM,
cc: @curryrasul we need this in zerokit since we're performing intermediate computations in the future subtree sharing strategy, would love a review and new release of pmtree to go with it :)
tia

[curryrasul]

@rymnc @seemenkina
That makes pmtree implementation too specific for one use-case. In general we don't want a public API for cache and all intermediate stuff ... On the other hand the change is too minor.

In general LGTM, I approve this, but I'd like to listen your thoughts on this @rymnc . Maybe we should create a pmtree fork for Vac, and here we should have it for a general case.

[rymnc]

sure, we can do that, is it okay if we publish to crates.io with the name vac-pmtree?

cc: @seemenkina