Add nullchar constant, NULLCHAR feature, and vectrex encoding

include/stdio.mfk

@@ -26,7 +26,7 @@ asm void putstrz(pointer hl) @$5550 extern
 void putstrz(pointer str) {
     byte index
     index = 0
-    while str[index] != 0 {
+    while str[index] != nullchar {
         putchar(str[index])
         index += 1
     }

include/string.mfk

@@ -25,7 +25,7 @@ word strz2word(pointer str) {
     errno = err_ok
     while true {
         char = str[i]
-        if char == 0 {
+        if char == nullchar {
             if i == 0 {
                 errno = err_numberformat
             }

include/string_fastindices.mfk

@@ -1,7 +1,7 @@
 byte strzlen(pointer str) {
     byte index
     index = 0
-    while str[index] != 0 {
+    while str[index] != nullchar {
         index += 1
     }
     return index
@@ -17,7 +17,7 @@ sbyte strzcmp(pointer str1, pointer str2) {
             if str1[i1] < str2[i2] { return -1 }
             return 1
         }
-        if str1[i1] == 0 {
+        if str1[i1] == nullchar {
             return 0
         }
         i1 += 1
@@ -33,5 +33,5 @@ void strzcopy(pointer dest, pointer src) {
         c = src[i]
         dest[i] = c
         i += 1
-    } while c != 0
+    } while c != nullchar
 }

include/string_fastpointers.mfk

@@ -3,7 +3,7 @@
 byte strzlen(pointer str) {
     pointer end
     end = str
-    while end[0] != 0 {
+    while end[0] != nullchar {
         end += 1
     }
     return lo(end - str)
@@ -11,8 +11,8 @@ byte strzlen(pointer str) {
 
 sbyte strzcmp(pointer str1, pointer str2) {
     while true {
-        if str1[0] == 0 {
-            if str2[0] == 0 {
+        if str1[0] == nullchar {
+            if str2[0] == nullchar {
                 return 0
             } else {
                 return -1
@@ -33,5 +33,5 @@ void strzcopy(pointer dest, pointer src) {
         dest[0] = c
         src += 1
         dest += 1
-    } while c != 0
+    } while c != nullchar
 }