Support surrounding begin and end

TODO

@@ -8,6 +8,10 @@
 - 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).
 
 # Shell
 - Persist the history.