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/V3CCtors.cpp

@@ -112,13 +112,14 @@ public:
             rootFuncp->name(m_basename);
             for (AstCFunc* const funcp : m_newFunctions) {
                 AstCCall* const callp = new AstCCall{m_modp->fileline(), funcp};
+                callp->dtypeSetVoid();
                 if (m_type.isClass()) {
                     callp->argTypes("vlSymsp");
                 } else {
                     if (m_type.isCoverage()) callp->argTypes("first");
                     callp->selfPointer("this");
                 }
-                rootFuncp->addStmtsp(callp);
+                rootFuncp->addStmtsp(callp->makeStmt());
             }
         }
     }