Require explicit types on implicits

[puffnfresh]

1. Enables better reasoning of implicits (i.e. you don't have to reason about inference, which is almost impossible in Scala)
2. Prevent things like Any or Nothing being inferred somewhere, which can really screw things up.
3. Prevents things like the following:

https://issues.scala-lang.org/browse/SI-8697

[copumpkin]

Seems closely related to #101

[stanch]

Careful with that axe! http://scalapuzzlers.com/#pzzlr-045

[copumpkin]

Pits of failure like that are exactly why this project exists!

[stanch]

@copumpkin By “this project”, do you mean Wartremover or Scalapuzzlers? Anyway, of course such unintentionally recursive implicits can be detected by this plugin as well. My point was that these two warts should go together to prevent potentially dangerous refactoring.

[copumpkin]

wart remover 😄

well, I can't say for sure why it exists, but shit like that is why I think the project is so valuable!

[puffnfresh]

@copumpkin this might be helpful:

Implicit search will never force inference of an implicit down lower in the same file as the implicit call site. This avoids spurious cycles in inference.

https://issues.scala-lang.org/browse/SI-8697?focusedCommentId=69897&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-69897

[ClaireNeveu]

This was added in #176