Assign variables

TODO

@@ -5,13 +5,15 @@
 - Parser should be able to collect errors.
 - Provide position information on parse tree nodes.
 - Move constants to the symbol table, so we can check at parse time for duplicates.
-- Allow defining variables.
 - Don't pass raw pointers to the visitor methods.
-- Make error abstract and derive unexpected_token in the lex module from it.
-- Wrap the tokens in a struct with methods for incrementing and lookups.
 - While loop.
 - If condition.
-- Grouping multiple statements with begin and end (compound_statement).
+- Introduce program node which contains global state and functions.
+- Calculate additional stack space needed for subexpressions in the allocator
+  visitor and not in the backend.
+- Support immediates greater than 12 bits.
+- It seems instructions are correctly encoded only if the compiler is running
+  on a little endian architecture.
 
 # Shell
 - Persist the history.