Do not manually check for ascii range

Clippy emits:

 error: manual check for common ascii range

As suggested, use `is_ascii_*` methods instead of checking for ascii
rang manually.

src/lib.rs

@@ -415,9 +415,9 @@ fn check_hrp(hrp: &str) -> Result<Case, Error> {
             return Err(Error::InvalidChar(b as char));
         }
 
-        if (b'a'..=b'z').contains(&b) {
+        if b.is_ascii_lowercase() {
             has_lower = true;
-        } else if (b'A'..=b'Z').contains(&b) {
+        } else if b.is_ascii_uppercase() {
             has_upper = true;
         };