Add if statements, unary minus, and fix GC safe points

- Lexer: recognize 'if' as keyword (TOK_IF)
- Parser: add NODE_IF with if_statement union, parse if/cond/body,
  handle unary minus in parse_term as 0 - expr
- Eval: add NODE_IF evaluation, move GC to NODE_BLOCK level to avoid
  destroying temporary values during sub-expression evaluation

projects/if.j

@@ -0,0 +1,4 @@
+x = 20
+if x < 10:
+    print x
+print -300

projects/while.j

@@ -1,4 +1,4 @@
 x = 0
-while x < 100000000:
+while x < 10000000:
     x = x + 1
 print x