Grammar update

1. Challenge should not be command.
2. Added Import
3. Added Front (make the turtle facing front)
4. Speed is already in the grammar. Added GetSpeed. (Why is there a
setspeed? speed itself is already the grammar for set)
5. Getx and Gety are now postfix expression (although not postfix, but
it has the same precedence as function). Getxy is still command_noarg
6. No change to print yet
7. LBRACKET and RBRACKET renamed to LBRACE and RBRACE to match the
context.