trace_fmt should support printing bitfields with leading zeros

[sandwichmaker]

Currently if you have a variable x that is a u8 and has value 3, printing it using trace_fmt!("{:b}",x) will print out 11. It would be nice to be able to say trace_fmt!("{:0b}",x) and get 00000011 instead.

This is particularly useful when debugging fixed point arithmetic.