Fix todo/error parsing

This was a bit more tricky than anticipated but played out nicely in
  the end. Now we have one holistic way of parsing todos and errors
  instead of it being duplicated between when/clause and sequence. The
  error/todo parser has been moved up to the expression part rather than
  being managed when parsing sequences. Not sure what motivated that to
  begin with.

  Fixes #621.

crates/aiken-lang/src/parser/definition/function.rs

@@ -27,7 +27,7 @@ pub fn parser() -> impl Parser<Token, ast::UntypedDefinition, Error = ParseError
             |((((public, name), (arguments, args_span)), return_annotation), body), span| {
                 ast::UntypedDefinition::Fn(ast::Function {
                     arguments,
-                    body: body.unwrap_or_else(|| UntypedExpr::todo(span, None)),
+                    body: body.unwrap_or_else(|| UntypedExpr::todo(None, span)),
                     doc: None,
                     location: ast::Span {
                         start: span.start,

crates/aiken-lang/src/parser/definition/test.rs

@@ -23,7 +23,7 @@ pub fn parser() -> impl Parser<Token, ast::UntypedDefinition, Error = ParseError
         .map_with_span(|(((fail, name), span_end), body), span| {
             ast::UntypedDefinition::Test(ast::Function {
                 arguments: vec![],
-                body: body.unwrap_or_else(|| UntypedExpr::todo(span, None)),
+                body: body.unwrap_or_else(|| UntypedExpr::todo(None, span)),
                 doc: None,
                 location: span_end,
                 end_position: span.end - 1,