Unneeded use of feature in fmt.rs

**Version**
1.14

**Platform**
Anything, it's a compile-time error

**Description**
Previously formatters (Display/Debug) used allocations for buffer. In 1.14 it's no longer true, but feature-config stayed in the implementation file https://github.com/recmo/uint/blob/9bc4c717fbe126dabaa722489a284021f404652f/src/fmt.rs#L2 but it's no longer needed. Is there any reason to keep it?