Clean up expression parser interactions with flags

[newmanrs]

The input parsing and behavior of the calculator with different arg flags in parse_expression and compute_rpn remains awkward. The function run_from_expr in the test suite, interactive and noninteractive modes call these functions slightly differently, but these should be unified. The clever hack of inserting print statements to implement verbose mode also causes kludges elsewhere when repeating last command (have to ignore prints). This could be removed just by making flags incl/ verbose part of the calculator state, and triggering on item.action() consumption.