Fixing float overflow during edge-flow computation in 'if' mapper (change to avoid dependence on the order of additions).

Example found in the 2007.03 Liberty Reference Manual that was also found
in the wild:

    input_voltage(CMOS) {
        vil : 0.3 * VDD ;
        vih : 0.7 * VDD ;
        vimin : -0.5 ;
        vimax : VDD + 0.5 ;
    }

Current implementation just parses the expression but no interpretation is done.

Restoring constraint manager to read old constraint file by default (use 'read_constr -n' to read new format).