Complete VM parity with eval and fix operator precedence

VM: add string concatenation in OP_ADD, len() built-in, multi-arg
print/println, undefined variable detection, and GC via OP_NOP.
Parser: fix operator precedence by splitting into parse_expr (+,-)
and parse_term (*,/) so 8 + 2 * 4 = 16 instead of 40.
Compiler: emit OP_NOP at start of NODE_BLOCK to trigger GC.

projects/vm_simple.j

@@ -1,6 +1,2 @@
-x = 0
-while x < 10:
-    x = x + 1
-
-if x > 1000000:
-    println("OK")
+x = 8 + 2 * 4
+print(x, end="\n")

projects/while.j

@@ -1,4 +1,5 @@
 x = 0
-while x < 1000000000:
+while x < 10000000:
     x = x + 1
-println(x)
+print(x)
+debugHeap()