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/V3Force.cpp

@@ -104,7 +104,7 @@ class ForceConvertVisitor final : public VNVisitor {
                 AstVarRef* const lhsp = new AstVarRef{flp, m_enVscp, VAccess::WRITE};
                 V3Number zero{m_enVscp, m_enVscp->width()};
                 zero.setAllBits0();
-                AstNodeMath* const rhsp = new AstConst{flp, zero};
+                AstNodeExpr* const rhsp = new AstConst{flp, zero};
                 AstAssign* const assignp = new AstAssign{flp, lhsp, rhsp};
                 AstActive* const activep = new AstActive{
                     flp, "force-init",