Add re-export of nom's IResult

[jonhoo]

Given that IResult is returned by parse_response, it would be useful for imap-proto to re-export nom::IResult (as well as perhaps the various types included in its variants) so that users of this crate do not need to explicitly pull in nom.

[djc]

Yeah -- I experimented with exporting a type alias called ParserResult to abstract over the nom dependency a little bit, but that did not seem to work so well because the enum variants don't seem to be accessible through the alias. They really should be to make it ergonomic to consume the result, so I'll need to do a bit more experimentation.

[djc]

I opened a Rust internals thread about this topic: https://internals.rust-lang.org/t/pre-rfc-enum-variants-through-type-aliases/6433.