Better job at inserting completions for higher-order methods

This little changes makes life easier when completing methods that take lambdas and
the call is in infix notation.

* use braces instead of parens
* insert a dummy lambda `x => ???` instead of `f`, or whatever the parameter name is

Fixed #1002250.
More tests and fix behavior for call-by-name parameters.
Merge pull request #787 from dragos/issue/completion-HOFs

Completion improvements for higher-order-functions