Use the mime crate for parsing the Accept header

[thomastaylor312]

Currently our code does some manual parsing, now that #153 is using the mime library for proper parsing, we should replace some of our own code with it in the SerializedData type

[technosophos]

Blocked on hyperium/mime#139

mime does not have TOML. (Right now, it is also missing Wasm, but there is a PR for that hyperium/mime#138)

Given the low rate of PR merges on that repo (one merge in 2020, none in 2021), we might just close this. If anyone knows @seanmonstar or any of the other Hyperium devs, maybe we should volunteer as co-maintainers on that repo. It's low activity, and getting more MIME types supported is definitely desirable.

[technosophos]

Okay, so on further clarification from @thomastaylor312, I am trying to switch over from strings to mime's types throughout Bindle. The above will not block this PR (though it would still be good to get application/toml into mime.