"Exchangable" versus "Exchangeable"

[atibaut]

#1041 proposes a fix (with deprecation) for the long-standing "Exchangable" misspelling in the fares model. Opinions needed if deprecation is really needed or can be a breaking fix.

[atibaut]

Update on the current decision in #1041:

1. The PR base moved from v2.0 to v2.1-wip, since v2.0 is stable and this is functional change.
2. The split approach stands for now. Names visible in instance documents (the nine elements, IsExchangable, the NameOfClass values) get correctly spelt twins and the misspelt forms stay as deprecated aliases, so no existing document breaks in 2.1. The type name ExchangableToEnumeration is renamed outright because type names never appear in instance documents.
3. The truly breaking step, removal of the deprecated misspelt forms, is proposed as a separate small PR against v3.0-wip.

So the question in this issue effectively becomes: should the breaking cleanup land in 3.0 as proposed, or is a breaking fix already acceptable in 2.1? Opinions welcome.