assignment: Add variable assignment

* Syntax for x = y
* Compiler creates Inst::Store thunks
* New Vm::store function

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/hello.np

@@ -1,3 +1,4 @@
 println(123);
 println(None);
-println("hell world");
+message = "hell world";
+println(message);