Be more explicit about AST types (#921)

This was born out of wanting FunctionDeclaration to explicitly have a Block for a body, and became a bit more of specifying types around. Note this forces exec_program to generate a Block for print()'s body, which is probably more correct as now we can expect a standard FunctionDeclaration AST structure, even for the built-ins.

There is a syntactic change here: a continuation's body is now a Block, not just a Statement. I've added an example disallowed test case. I think this is more reasonable syntax.

Other than that, note that optional_else now generates a valid Block. This has me thinking about whether we can eliminate Sequence, but that seemed well out of scope for this.

executable_semantics/ast/expression.cpp

@@ -30,7 +30,7 @@ auto ExpressionFromParenContents(
 
 auto TupleExpressionFromParenContents(
     Nonnull<Arena*> arena, SourceLocation source_loc,
-    const ParenContents<Expression>& paren_contents) -> Nonnull<Expression*> {
+    const ParenContents<Expression>& paren_contents) -> Nonnull<TupleLiteral*> {
   return arena->New<TupleLiteral>(source_loc, paren_contents.elements);
 }