Centralise error positioning + cover custom de errors

[juntyr]

Fixes #203 and supersedes #209. Also includes some small minor refactoring of the ErrorCode names.

In particular, the Deserializer now uses ErrorCode as its error type, no longer having to pretend to produce error positions in all cases (which as #209 showed were missing in all serde-internal and user-generated cases, and can only be added during deserialization to the former). Instead, the exposed helper functions from_str etc. now add the positional information afterwards, which cleans up the code and gives us positions for custom errors in serde-code and user-code for free (though the position still only points at the character after the error or directly at it).

• I've included my change in CHANGELOG.md

[juntyr]

?r @torkleyy @kvark

[juntyr]

There is also the question whether ron should split the error type into two, one for ser and one for de, or keep the combined one (like serde_json). What are your thoughts? For ser it seems like we only generate Io and Message errors that are never positioned anyways (unless we wanted to arbitrarily track a fake line and column somehow).

[juntyr]

?r @torkleyy Any thoughts?

[kvark]

I'm very sorry that neither me or torkley are responsive!
If you are confident with the move, please feel free to commit without reviews if N days passed without any feedback (as long as this deadline is visible). For this PR, I'll review it now.