`impl Default for SerializedSignature` doesn't seem to make sense

[Kixunil]

It's all zeroes, thus invalid signature. It's used in the implementation which could use MaybeUninit to avoid zeroing or, if we're too paranoid for that, there could be an internal constant ZEROED. If there's anyone actually using zeroed signatures we could make it pub.

[apoelstra]

Oops! This dates back to #100 and was probbaly an oversight. It shouldn't exist.