exec_node: Add new talloc-based new()

And fix all callers to use the tallbac-based new for exec_node
construction. We make ready use of talloc_parent in order to get
valid, (and appropriate) talloc owners for everything we construct
without having to add new 'ctx' parameters up and down all the call
trees.

This closes the majority of the memory leaks in the
glsl-orangebook-ch06-bump.frag test:

	total heap usage: 55,623 allocs, 42,672 frees
	(was 14,533 frees)

Now 76.7% leak-free. Woo-hoo!

s_expression.cpp

@@ -49,7 +49,7 @@ s_list::length() const
 }
 
 static s_expression *
-read_atom(const char *& src)
+read_atom(void *ctx, const char *& src)
 {
    char buf[101];
    int n;
@@ -65,20 +65,20 @@ read_atom(const char *& src)
       int i = strtol(buf, &int_end, 10);
       // If strtod matched more characters, it must have a decimal part
       if (float_end > int_end)
-	 return new s_float(f);
+	 return new(ctx) s_float(f);
 
-      return new s_int(i);
+      return new(ctx) s_int(i);
    }
    // Not a number; return a symbol.
-   return new s_symbol(buf);
+   return new(ctx) s_symbol(buf);
 }
 
 s_expression *
-s_expression::read_expression(const char *&src)
+s_expression::read_expression(void *ctx, const char *&src)
 {
    assert(src != NULL);
 
-   s_expression *atom = read_atom(src);
+   s_expression *atom = read_atom(ctx, src);
    if (atom != NULL)
       return atom;
 
@@ -87,10 +87,10 @@ s_expression::read_expression(const char *&src)
    if (sscanf(src, " %c%n", &c, &n) == 1 && c == '(') {
       src += n;
 
-      s_list *list = new s_list;
+      s_list *list = new(ctx) s_list;
       s_expression *expr;
 
-      while ((expr = read_expression(src)) != NULL) {
+      while ((expr = read_expression(ctx, src)) != NULL) {
 	 list->subexpressions.push_tail(expr);
       }
       if (sscanf(src, " %c%n", &c, &n) != 1 || c != ')') {