Do not 0 pad on debug and hex formatting

[onbjerg]

Is your feature request related to a problem? Please describe.

Currently ruint types add leading 0s in the debug and hex formatting impls, but Rust's own primitive types do not do this.

If you want leading zeros, you can use the padding built in to the Rust formatter

Describe the solution you'd like

Remove leading zeros

[gakonst]

@recmo wdyt here? we're happy to fix

[prestwich]

related, padding directives are not currently supported https://github.com/recmo/uint/issues/295