std::fmt: minor optimization for the Fprint function

std/fmt/print.jule

@@ -11,13 +11,15 @@ use std::internal::strings::{StrBuilder}
 // Prints arguments to file by default formatting.
 // See documentation of format function for formatting.
 fn Fprint(mut f: &File, args: ...any) {
-    mut buf := StrBuilder.New(20)
+    mut sb := StrBuilder.New(20)
     for _, arg in args {
-        fmt::FmtByDefault(buf, arg)
-        f.Write(unsafe { buf.Buf() }) else {
+        fmt::FmtByDefault(sb, arg)
+        f.Write(unsafe { sb.Buf() }) else {
             panic("std::fmt: Fprint: error occurs when printing")
         }
-        buf.Clear()
+        // Do not use the [Clear] method to avoid making new allocations.
+        // The buffer used temporarily, so just clear the length, not capacity.
+        unsafe { sb.SetBuf(sb.Buf()[:0]) }
     }
 }