Get closures working!

In the process, I had to change the grammar. There is now a strong
separation between statements and expressions. The code was just wrong
before when it popped locals at the end of a block scope because there
could be temporaries on the stack if the block was in expression
position. This fixes that.

Still need to implement closing over `this`.

test/variable/scope_if.wren

@@ -1,9 +1,13 @@
 // Create a local scope for the 'then' expression.
 var a = "out"
-if (true) var a = "in"
+if (true) {
+  var a = "in"
+}
 io.write(a) // expect: out
 
 // Create a local scope for the 'else' expression.
 var b = "out"
-if (false) "dummy" else var b = "in"
+if (false) "dummy" else {
+  var b = "in"
+}
 io.write(b) // expect: out