Switch to new scheme for translating context bounds

[odersky]

This is a trial balloon to find out what would break if we change the desugaring of context bounds to the one currently supported under -source future. Context bounds now translate to using clauses instead of implicit parameters.

[odersky]

Conclusion: lots of things break, currently, if we try to do that. We'll need a deprecation phase first, where we warn when an implicit parameter stemming from a context bound is instantiated with an explicitly passed, plain (non-using) argument.

The question is, from which version should we deprecate? The problem is is there's no good way to avoid the deprecation in code that still should compile under Scala 2, since Scala 2 has no equivalent for a using argument clause.

So, what is more important for a Scala 3 LTS? Being able to cross-compile with Scala 2 without deprecation warnings, or getting deprecation warnings early to know what code should be changed in Scala 3?

[smarter]

We could update the Scala 2 compiler to accept but ignore the using keyword (ideally by default but possibly under -Xsource:3), just like I previously tweaked it to accept but ignore case in pattern bindings (scala/scala#9721) and open/infix (scala/scala#9580).

[odersky]

I think that would be the best course of action. I'll open an issue.