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

include/verilated.cpp

@@ -395,7 +395,7 @@ void _vl_debug_print_w(int lbits, const WDataInP iwp) VL_MT_SAFE {
 }
 
 //===========================================================================
-// Slow math
+// Slow expressions
 
 WDataOutP _vl_moddiv_w(int lbits, WDataOutP owp, const WDataInP lwp, const WDataInP rwp,
                        bool is_modulus) VL_MT_SAFE {

include/verilated_funcs.h

@@ -929,7 +929,7 @@ static inline int _vl_cmps_w(int lbits, WDataInP const lwp, WDataInP const rwp)
 }
 
 //=========================================================================
-// Math
+// Expressions
 
 // Output NOT clean
 static inline WDataOutP VL_NEGATE_W(int words, WDataOutP owp, WDataInP const lwp) VL_MT_SAFE {
@@ -1917,7 +1917,7 @@ static inline WDataOutP VL_SEL_WWII(int obits, int lbits, WDataOutP owp, WDataIn
 }
 
 //======================================================================
-// Math needing insert/select
+// Expressions needing insert/select
 
 // Return QData from double (numeric)
 // EMIT_RULE: VL_RTOIROUND_Q_D:  oclean=dirty; lclean==clean/real