style(core/rust): less expensive conversion from known good utf8 string

core/embed/rust/src/trezorhal/slip39.rs

@@ -1,3 +1,4 @@
+use core::str;
 use cstr_core::CStr;
 
 use super::ffi;
@@ -25,6 +26,6 @@ pub fn button_sequence_to_word(prefix: u16) -> Option<&'static str> {
     } else {
         // SAFETY: On success, `button_sequence_to_word` should return a 0-terminated
         // UTF-8 string with static lifetime.
-        Some(unsafe { CStr::from_ptr(word).to_str().unwrap_unchecked() })
+        Some(unsafe { str::from_utf8_unchecked(CStr::from_ptr(word).to_bytes()) })
     }
 }