[ENH] A functional statistics-computing method. (#5735)

## Description of changes

This includes a new function that can be attached to count terms in
keys.

## Test plan

Demo.  Will add tests once it looks good.

## Migration plan

N/A

## Observability plan

N/A

## Documentation Changes

N/A

Author: rescrv

rust/worker/src/execution/functions/mod.rs

@@ -0,0 +1,3 @@
+mod statistics;
+
+pub use statistics::StatisticsFunctionExecutor;