Restrict implicit conversions to implicit method or ImplicitConverter instance

[adriaanm]

See: dotty

[smarter]

@dwijnand I think the relevant PR for this issue is scala/scala3#2065, implicit function types are a different beast.

[smarter]

It'd be great to get ImplicitConverter in the 2.13 standard library to make cross-compilation with Dotty easier.

[nafg]

Then can we have <% back?