issue2527 character set data type

[winitzki]

There are lots of changes if we want to implement #2527

The changes are straightforward and perhaps beneficial because:

• It becomes clearer in the code that we assume auto-detected character set in certain places, or that we are dealing with character sets that may be auto-detected or overridden.
• The dependence of dhall-lsp-server and openapi-to-dhall on the private CharacterSet structure has become apparent and needs to be refactored.