get the sum of all pairs_count

[ExevilRMKH]

Greetings!

I am Sbroad developer in Picodata project (https://git.picodata.io/picodata/picodata/sbroad). We use your parser fr work in SQL. Our colliage made MR in your project (#965).

Short:
To optimize memory usage and algorithm running time, we would like to know the number of all pairs in the childs, so we can plan the size of the vector we needed to work wth the tree.

What we want:
package(https://github.com/pest-parser/pest/blob/master/pest/src/iterators/pairs.rs) In the Pairs.rs file there is a variable pairs_count which contains the number of children. It would be nice to be able to also store information about "all_pairs_count", which would include the sum of all "pairs_counts" from "root" to "last_pairs"