Ths commit adds a new `IDNameLibMap` to `Main`, used during file reading
to quickly find already read linked IDs.
Without that, search would use string-based search over list of linked
data, which becomes extremely slow and inneficient in cases where a lot
of IDs are linked from a same library. See also {T89194}.
Extrem-usecase reported in T89194 is now about 4 times faster in linked
data reading (about 2 times faster for the whole .blend file loading).
More normal cases (like Sprites studio production files) have barely
measurable speed improvements, a few percents at best.
NOTE: `main_idmap` API was extended to support insertion and removal of
IDs from the mapping, avoids having to re-create the whole thing several
time during libraries expansion in readcode.
Differential Revision: https://developer.blender.org/D11757
db4fe8e322