Completion improvements for higher-order-functions

This is built on top of #786, and the last code contribution from the plane :)

Better job at inserting completions for higher-order methods

This little changes makes life easier when completing methods that take lambdas.

* use braces instead of parens
* insert a dummy lambda `x => ???` instead of `f`, or whatever the parameter name is

Fixed #1002250.
When you are already at it, you could refactor this `StringBuffer` away. `explicitParamNames.iterator.map(_.mkString("(", ", ", ")")).mkString(completion, "", "")` should do the same and should also have the same performance.