Add block scoping and statement/expression distinction

Implement strict block scoping and distinguish between statements and
expressions.
This change introduces hierarchical scopes for functions and blocks,
enforces that `def` is a statement with no return value, and prevents
statements from appearing in expression positions or as the last element
of a block. A new example `design-flaw.myc` is added to demonstrate
the uninitialized variable issue solved by these changes.

examples/design-flaw.myc

@@ -1,7 +1,7 @@
 
 (do
   (do
-    (def x "hh")
+    (if false (def x "hh"))
   )
   (print x)
 )