Exercise: calculator

Port of an older exercise with reworked structure that addresses issues I've been running into with groups in past two years.

The help text and a step by step solution promote the use of `?` as a default error handling option.

A step by step solution also mentions Rust Analyzer commands that people new to Rust would greatly benefit from.

Author: listochkin

exercise-book/src/SUMMARY.md

@@ -10,6 +10,7 @@
 - [Fizzbuzz with match](./fizzbuzz-match.md)
 - [Rust Latin](./rustlatin.md)
 - [URLs, match, result](./urls-match-result.md)
+- [Calculator](./calculator.md)
 - [Iterators](./iterators.md)
 - [SimpleDB](./simple-db.md)
   - [Knowledge](./simple-db-knowledge.md)