Add arithmetic expressions (SET /a)

[niutech]

Please add SET /a command line switch with mathematical expressions, using e.g. TinyExpr, expr or expression_parser.

[PerditionC]

I looked into using https://github.com/zserge/expr - while it compiles with OW with minimal changes (declaring and initializing variables is done at use instead of start of scope). This will require some reworking the build as adding it causes the segments to exceed 64KB. Leaving as future enhancement project.

[niutech]

@PerditionC Thanks for looking at it! Expr uses float and all operators, while set uses only integers and + - * / and bitwise operators. Maybe if you strip down expr to basic calculations and make it use int, it would fit in 64KB?