java functional expressions: render functional expressions in show implementation using stubs if possible

java functional expressions: remove possibly expensive toString() in PsiLambdaExpressionImpl and PsiMethodReferenceExpressionImpl (e.g. show implementation + JList)

For instance, when inlining single-quoted string inside triple-quoted
string with the same type of quotes.

* See PyProjectSynchronizer for entry point
* DownloadAction refactored to extract  download
* VagrantSupportImpl refactored to fetch mapped folders

before attempting to load its contents

function expression searcher: find most likely files only among of files where occurrences was found. method names should be found in sam-class use-scope (follow-up 4c72b12c)

Partially reverted equals/hashCode changes.