Calculator should allow variables

[skx]

The calculator currently allows either:

• sysbox calc '3 + 5 * 6'
  • Run the expression, and show the result.
• sysbox calc
  • Run a REPL

The latter case should allow variables. Something like:

rent = 710
rent * 6  / 12

This does mean dropping the current go-based scanner/lexer/evaluator. But writing a calculator is a trivial thing, even with precedence and variables.

[skx]

I've hacked up a simple calculator which supports variables, albeit with a let x = prefix.

It handles +, -, *, /, with floating point, and appropriate precedence.

TODO:

• Support parenthesis.