Fix implicit casts in assignments (part 3).

This patch adds support for bool/bit vector types on the LHS of
a parameter declaration and ensures implicit casts in parameter
declarations are performed where necessary.
Fix implicit casts in assignments (part 4).

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).
Fix implicit casts in assignments (part 5).

This patch removes some code that has been made redundant by the
more general handling of implicit casts.
Fix implicit casts in assignments (part 6).

IVL_VT_NO_TYPE is now used to signal an untyped LHS in a parameter
declaration. The parser function that handles specparam declarations
needs to do this too. Also, although it should never happen, make
sure we don't set the expression width in a NetECast object to a
negative number. Make constant evaluation of NetECast objects
observe the expression width.