rusty-gd: Pretty print enum variants.

Implements the fmt::Display trait for all enums.

It will print a particular enum variant in hex followed by the name of
the variant in parantheses. Example showing the reset opcode.

0x0C03 (RESET)

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost DirectHciTest

Change-Id: I60d62a70890ab2daa51e206b13ee189a1468cbd0