Conditional expression parsing is ostensibly implemented, but since it
depends on the rest of the expression parsers -- not yet implemented --
it cannot be tested in isolation.
Also includes an initial implementation of the conditional expression
node, but this is also not yet tested and so may need further revisions
once we're in a better position to test it.
The implementation of Variables will be identical for every Expression
implementation since we just wrap our AST-walk-based "Variables" function
to do the work.
Rather than manually copy-pasting the declaration for each expression
type, instead we'll generate this programmatically using "go generate".
This will need to be re-run each time a new expression node type is
added, in order to make it actually implement the Expression interface.
