replace manual 'ch > 0x7f' checks with is_ascii

Author: apoelstra

src/descriptor/tr.rs

@@ -458,7 +458,7 @@ impl<Pk: MiniscriptKey> fmt::Display for Tr<Pk> {
 // Helper function to parse string into miniscript tree form
 fn parse_tr_tree(s: &str) -> Result<Tree, Error> {
     for ch in s.bytes() {
-        if ch > 0x7f {
+        if !ch.is_ascii() {
             return Err(Error::Unprintable(ch));
         }
     }

src/expression.rs

@@ -213,7 +213,7 @@ impl<'a> Tree<'a> {
         // Filter out non-ASCII because we byte-index strings all over the
         // place and Rust gets very upset when you splinch a string.
         for ch in s.bytes() {
-            if ch > 0x7f {
+            if !ch.is_ascii() {
                 return Err(Error::Unprintable(ch));
             }
         }