this is a breaking change in their behaviour:
- if they receive an empty input, they will return incomplete (if not at
eof)
- if they reach the end of input without the child parser returning an
error, they will return Incomplete
while this may break some parsers, this behaviour is more logical with
the rest of nom and much stricter
this will make it easier to be compatible with other crates like
error_chain, and we can reuse a lot of code coming for free with
Result. Incomplete is merged into the error side. I still do not
consider it to be an error,but this side will also contain unrecoverable
errors at some point (errors that cause alt and others to return
instead of testing the next branch), so it will be easier to put it in
this enum.
