`expr`: optimization for integer values

[tertsdiepraam]

Currently, expr uses only string values internally. This means that for an expression like 1 + 2 + 9 is evaluated as follows:

1. Get the strings "1" and "2"
2. Parse them into integers 1 and 2
3. Add them to get 3
4. Turn 3 into the string "3"
5. Parse "3" back into the integer 3
6. Parse "9" into 9
7. Add them to get 12
8. Turn 12 into the string "12"

That's a lot of parsing and stringifying. Instead we could have an enum that can contain either an integer or a string which can be turned into the desired value.