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/astgen

@@ -1196,7 +1196,7 @@ def write_dfg_dfg_to_ast(filename):
                 "void visit(Dfg{t}* vtxp) override {{\n".format(t=node.name))
             for i in range(node.arity):
                 fh.write(
-                    "    AstNodeMath* const op{j}p = convertSource(vtxp->source<{i}>());\n"
+                    "    AstNodeExpr* const op{j}p = convertSource(vtxp->source<{i}>());\n"
                     .format(i=i, j=i + 1))
             fh.write(
                 "    m_resultp = makeNode<Ast{t}>(vtxp".format(t=node.name))