Migrate loop status from blocks into libdata

Blocks will soon need to be shared across parsers. Migrate the loop status
(like break or continue) from the block into the libdata. It turns out we
only ever need one, we don't need to track this per-block.

Make it an enum class.

src/parser.cpp

@@ -138,8 +138,6 @@ void parser_t::push_block_int(block_t *new_current) {
         new_current->skip = false;
     }
 
-    new_current->loop_status = LOOP_NORMAL;
-
     // Push it onto our stack. This acquires ownership because of unique_ptr.
     this->block_stack.emplace_back(new_current);