Implement strict block scoping and distinguish between statements and
expressions.
This change introduces hierarchical scopes for functions and blocks,
enforces that `def` is a statement with no return value, and prevents
statements from appearing in expression positions or as the last element
of a block. A new example `design-flaw.myc` is added to demonstrate
the uninitialized variable issue solved by these changes.
Michael Schimmel
3b063dc2c9