Implement functions

- Implement function declaration and call
- Change the precalculated variable stack positions to contain the
  offset from the end instead of the absolute position. This is
  important for passing fun args on the stack
- Add the ability to offset the stackframes. This is used to delete the
  stack where the fun args have been stored before the block executes
- Implement exit type for blocks in interpreter. This is used to get the
  return values and propagate them where needed
- Add recursive fibonacci examples

examples/recursive_fib.nek

@@ -0,0 +1,9 @@
+fun fib(n) {
+    if n <= 1 {
+        return n;
+    } else {
+        return fib(n-1) + fib(n-2);
+    }
+}
+
+print fib(30);

examples/recursive_fib.py

@@ -0,0 +1,6 @@
+def fib(n):
+    if n <= 1:
+        return n
+    return fib(n-1) + fib(n-2)
+
+print(fib(30))