Merge #312: Small optimization of Tr descriptor parsing.

00e40e8 Remove double scan for checksum in FromStr (Jeremy Rubin)

Pull request description:

ACKs for top commit:
  sanket1729:
    utACK 00e40e8

Tree-SHA512: 845e724340e70d3d1d5672a8a61bd9f3857328d830cecc33997cece57033ea814b4e5fead702dd3f166703702995ec22e226aef968fd711f0215631b0bd3ed30

Author: sanket1729

src/descriptor/mod.rs

@@ -739,12 +739,13 @@ where
     type Err = Error;
 
     fn from_str(s: &str) -> Result<Descriptor<Pk>, Error> {
-        let desc_str = verify_checksum(s)?;
         // tr tree parsing has special code
-        if desc_str.starts_with("tr") {
-            let tr = Tr::from_str(desc_str)?;
-            Ok(Descriptor::Tr(tr))
+        // Tr::from_str will check the checksum
+        // match "tr(" to handle more extensibly
+        if s.starts_with("tr(") {
+            Ok(Descriptor::Tr(Tr::from_str(s)?))
         } else {
+            let desc_str = verify_checksum(s)?;
             let top = expression::Tree::from_str(desc_str)?;
             expression::FromTree::from_tree(&top)
         }