Convert dwarfdump to use Reader

[philipc]

The main change is the addition of to_slice, to_string and to_string_lossy to the Reader trait. Note that these functions return values with a lifetime of the Reader, not of the underlying data. If required, we could add a lifetime parameter to the trait, but this will propagate through all uses of the trait, so I'm not keen to do that unless there is a need, and I think it would be hard to take advantage of this in a generic way. As it is, you can use EndianBuf explicitly if that's what you need.

[coveralls]

Coverage decreased (-0.004%) to 93.527% when pulling d822c9c on philipc:dwarfdump into e042865 on gimli-rs:master.

[fitzgen]

Looks great :)