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/V3Combine.cpp

@@ -69,7 +69,9 @@ class CombineVisitor final : VNVisitor {
             if (funcp->emptyBody()) {
                 // Delete call sites
                 for (AstCCall* const callp : m_callSites(funcp)) {
-                    VL_DO_DANGLING(callp->unlinkFrBack()->deleteTree(), callp);
+                    UASSERT_OBJ(VN_IS(callp->backp(), StmtExpr), callp,
+                                "Deleting non-statement call");
+                    VL_DO_DANGLING(callp->backp()->unlinkFrBack()->deleteTree(), callp);
                 }
                 m_callSites(funcp).clear();
                 // Remove from list