T2345: Replace instances of NULL with nullptr

The following changes have been applied to src and lib folders:
regex replace: ([^"_])NULL ⇒ \1nullptr (8968 chgs in src, 1153 in lib)
Manually find/change: (void \*) nullptr ⇒ nullptr (1 case)
regex find: ".*?nullptr.*?"
  Manually ~14 cases back to "NULL" in src, ~2 in lib
  regex finds a few false positive where nullptr appears between two
  strings in a function call

src/variable.h

@@ -93,9 +93,9 @@ class Variable : protected Pointers {
     Tree **extra;          // ptrs further down tree for nextra args
 
     Tree() :
-      array(NULL), iarray(NULL), barray(NULL),
+      array(nullptr), iarray(nullptr), barray(nullptr),
       selfalloc(0), ivalue1(0), ivalue2(0), nextra(0),
-      first(NULL), second(NULL), extra(NULL) {}
+      first(nullptr), second(nullptr), extra(nullptr) {}
   };
 
   int compute_python(int);