Major rewrite of indexed part selects.

This patch is a major rewrite of the indexed part selects (+: and -:).

It made the following enhancements:

1. Make indexed part selects work correctly with both big and little
   endian vectors.

2. Add a warning flag that warns about constant out of bounds/or 'bx
   indexed selects.

3. Moved the -: parameter code to its own routine.

4. Added support for straddling before part selects in a CA.

5. Added more assert(! number_is_unknown) statements.

6. Add warning for &PV<> select with a signed index signal that is
  less than the width of an int. This will be fixed later.

7. Add support for loading a 'bx/'bz constant into a numeric register.

8. Add a number of signed value fixes to the compiler/code generator.

9. Major fix of draw_select_expr() in the code generator.

PExpr.h

@@ -339,6 +339,12 @@ class PEIdent : public PExpr {
 					   NetScope*found,
 					   const NetExpr*par_msb,
 					   const NetExpr*par_lsb) const;
+      NetExpr*elaborate_expr_param_idx_do_(Design*des,
+					   NetScope*scope,
+					   const NetExpr*par,
+					   NetScope*found,
+					   const NetExpr*par_msb,
+					   const NetExpr*par_lsb) const;
       NetExpr*elaborate_expr_net(Design*des,
 				 NetScope*scope,
 				 NetNet*net,