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.
The `Analyzer` is now responsible for determining the purity of `Define`
bindings. Previously, this responsibility was left to the caller.
Added a new example `design-flaw.myc` to test this change.
Updated `soa_series.myc` due to the purity change.
Michael Schimmel