Fix zero/sign extension of parameter values in expressions.

For sized parameters, the choice of zero or sign extension should
be based on the expression type, not the parameter type.
Add -compatible command line option to vvp.

This option is intended to make it easier to compare results from
Icarus with results from other simulators. For now, the only effect
it has is to change the default format for displaying real numbers
when no format string is supplied.
Improved error message.
Extend -gstrict-expr-width to cover unsized numbers.

To be strictly compliant with the standard and compatible with other
EDA tools, unsized numbers should be treated as having a fixed size
(the same size as an integer). The -gstrict-expr-width option is
extended to allow the user to enable this behaviour.