Math operators (#1601)

* Add some math operations

* WIP for adding compound expressions

* precedence parsing

* paren expressions

* better lhs handling

* add compound comparisons and shorthand lefthand parsing

* Add or comparison and shorthand paths

crates/nu-cli/tests/commands/mod.rs

@@ -18,6 +18,7 @@ mod insert;
 mod last;
 mod lines;
 mod ls;
+mod math;
 mod mkdir;
 mod mv;
 mod open;