Remove rayon_croissant and clean up contains_floats (#29960)

Remove rayon_croissant and refactor the way that information about
floats in flows bubbles up. This simplifies the code a good deal and
lets us take advantage of some more optimized functions provided by
rayon. This removes 2 crates from the dependency tree.

In addition, this allows avoiding passing `contains_floats` up from
every box tree construction function. This makes things simpler, but
also opens up the possibility of passing more of these flags up in the
future (such as `contains_counters`).

components/layout_2020/flow/inline.rs

@@ -39,6 +39,7 @@ pub(crate) struct InlineFormattingContext {
     // Whether this IFC contains the 1st formatted line of an element
     // https://www.w3.org/TR/css-pseudo-4/#first-formatted-line
     pub(super) has_first_formatted_line: bool,
+    pub(super) contains_floats: bool,
 }
 
 #[derive(Debug, Serialize)]
@@ -172,6 +173,7 @@ impl InlineFormattingContext {
             inline_level_boxes: Default::default(),
             text_decoration_line,
             has_first_formatted_line,
+            contains_floats: false,
         }
     }