Missing code for `set_blocked`

[PierreGuyot]

rustrogueliketutorial/book/src/chapter_57a.md

Line 133 in 33872fe

In `spatial/mod.rs`, add the `index_entity` function:

There is not text describing the code for set_blocked which is used in the snippet above. It's straightforward to implement and gets removed later in the chapter, but I think it should be added for completeness/clarity (as it's additional confusion to have to search and check if you missed something while reading the chapter).