GenericRAII stores only a reference to cleanup function

[MatthiasKillat]

Refactoring

1. Pass a cleanup function as function_ref instead of function (but store as function as is).
2. Rename into ScopeGuard

cf. https://stackoverflow.com/questions/31365013/what-is-scopeguard-in-c

[MatthiasKillat]

Actually the cleanup function is correct on master (and a cxx function). We should still consider renaming GenericRAII to ScopeGuard.

[elBoberido]

@MatthiasKillat yes, the ScopeGuard terminology is already used by the C++ community