Make all expressions derive from AstNodeExpr (#3721).

Apart from the representational changes below, this patch renames
AstNodeMath to AstNodeExpr, and AstCMath to AstCExpr.

Now every expression (i.e.: those AstNodes that represent a [possibly
void] value, with value being interpreted in a very general sense) has
AstNodeExpr as a super class. This necessitates the introduction of an
AstStmtExpr, which represents an expression in statement position, e.g :
'foo();' would be represented as AstStmtExpr(AstCCall(foo)). In exchange
we can get rid of isStatement() in AstNodeStmt, which now really always
represent a statement

Peak memory consumption and verilation speed are not measurably changed.

Partial step towards #3420

src/V3Assert.cpp

@@ -105,7 +105,7 @@ private:
                  // This allows syntax errors and such to be detected normally.
                  (v3Global.opt.assertOn()
                       ? static_cast<AstNode*>(
-                          new AstCMath{fl, "vlSymsp->_vm_contextp__->assertOn()", 1})
+                          new AstCExpr{fl, "vlSymsp->_vm_contextp__->assertOn()", 1})
                       : static_cast<AstNode*>(new AstConst{fl, AstConst::BitFalse{}}))),
             nodep};
         newp->isBoundsCheck(true);  // To avoid LATCH warning