remove scalac dependency

[yanns]

I'm not sure if we need the scalac dependency in core.

[fthomas]

The 2.12 build failed with

[error] /home/travis/build/fthomas/refined/modules/core/jvm/target/scala-2.12/src_managed/test/eu/timepit/refined/autoDoctest.scala:22:43: Class scala.tools.nsc.typechecker.Contexts not found - continuing with a stub.
[error]     val x: Int Refined Greater[W.`5`.T] = 100
[error]                                           ^

We've seen this error before in #256 which was also caused by removing the scala-compiler dependency in version 0.7.0. We brought it back in 0.8.0.

The dependency is needed for refined's macros. If we want to remove that dependency, we have to move the macros into a new refined-macros module. Moving macros into its own module would make it easier to cross-compile refined with dotty, so I'd be in favour of such split.

[fthomas]

I've opened an issue for moving the macros to a new module and hence closing this PR.