stop dumping pratt into combinator namespace

until we find a satisfactory api

based on
winnow-rs#614 (comment)

> "We are dumping a lot of stray types into combinator. The single-line
summaries should make it very easy to tell they are related to
precedence"

src/combinator/mod.rs

@@ -164,10 +164,9 @@ mod core;
 mod debug;
 mod multi;
 mod parser;
-mod precedence;
 mod sequence;
 
-mod shunting_yard;
+pub mod precedence;
 
 #[cfg(test)]
 mod tests;