Avoid memory allocations in assignments to interfaces

[lubkoll]

From @lubkoll on November 13, 2016 22:48

Avoid memory allocations if an object of same type is already hold.

Copied from original issue: spacy-dev/Spacy2#4

[lubkoll]

Probably needs to wait until finish of porting friendly-type-erasure to C++ with clangTooling

[lubkoll]

Only is of interest for copying of pointers (where instead the content can be copied if both containers contain objects of same type).