Implement Show typeclass

The goal is to automatically provide a better toString for case classes with field names.

Example :

case class Person(name: String, age: Int)
implicit personShow = ... // calling macro here
val p = Person("bob", 40)
Show.show(p) // returns  "Person(name = bob, age = 40)"
//or 
// import implicit class
p.show // returns  "Person(name = bob, age = 40)"

[oraclewalid]

To use the macro from external project, I think user must add addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M10" cross CrossVersion.full) to build.sbt, then external projet can call ShowMacro

[loicdescotte]

Remaining tasks for a first version of Show (using show branch) :

• make the macro usable from the tests (should work for any client of the lib too)
• move the case classes outside the Show.scala file (move to the right test)
• move show tests at the same level as other tests