Move new to top of impl block

To ease reading, put the `new` method at the top of the impl block.
Improve rustdocs while we do it.

src/util/bip158.rs

@@ -110,17 +110,17 @@ impl FilterHash {
 }
 
 impl BlockFilter {
+    /// Creates a new filter from pre-computed data.
+    pub fn new (content: &[u8]) -> BlockFilter {
+        BlockFilter { content: content.to_vec() }
+    }
+
     /// compute this filter's id in a chain of filters
     pub fn filter_header(&self, previous_filter_header: &FilterHeader) -> FilterHeader {
         let filter_hash = FilterHash::hash(self.content.as_slice());
         filter_hash.filter_header(previous_filter_header)
     }
 
-    /// create a new filter from pre-computed data
-    pub fn new (content: &[u8]) -> BlockFilter {
-        BlockFilter { content: content.to_vec() }
-    }
-
     /// Compute a SCRIPT_FILTER that contains spent and output scripts
     pub fn new_script_filter<M>(block: &Block, script_for_coin: M) -> Result<BlockFilter, Error>
         where M: Fn(&OutPoint) -> Result<Script, Error> {