Change selectReader to use total function

[enlait]

We use -Xfatal-warnings + -Xlint:unsound-match, which is good for preventing runtime MatchErrors on incomplete matches on sealed subtypes.

However, selectReader accepts a partial function.

1. "partial" property is not used withing the code, PF is just applied to receive a reader
2. total function heuristics for emitting warnings are not applied to selectReader argument

I think that partial argument, even if implemented to actually be useful for something, shouldn't be the default.