This patch implements the evaluate_function method for the NetDisable
and NetSTask classes. It also makes the checks for a function being
constant work when the function contains nested scopes (named blocks).
This patch fixes a few issues/bugs that showed up when testing the
fixes for implicit casts:
1. Make the compile-time implementation of $abs, $min, and $max match
the run-time behaviour (system functions can't be polymorphic).
2. Correctly set the type (signed/unsigned) of the result of an
assignment inside a constant user function (the LHS should not
inherit the type of the RHS).
3. Fix a bug in the verinum(double) constructor (insufficient bits
were allocated in the case where the double value rounded up to
the next power of two).
This patch implements the evaluate_function method for the NetEBBits,
NetEBDiv, NetEBLogic, NetEBMinMax, and NetEBPow classes. It also
factors out some common code into the NetEBinary class.