Use new Main->relations ID usages mapping in BKE_library_make_local().
This allows a noticeable simplification in code, and can be up to twice
quicker as previous code (Make Local: All from 2 to 1 minute e.g. in a
huge production file with thousands of linked data-blocks).
Note that new code has been successfuly tested with several complex cases
(production files from Agent327), as well as some testcases from recent
bug reports related to that function. But as always, nothing beats real
usage by real users, so please check this before we release 2.79. ;)
Main areas that would be affected: Make Local operations (L shortcut in
3DView), and append from libraries.
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