Extract 'coll' library from 'core'

Core is supposed to be the most basic library, providing only
really common features used everywhere like fiber, diag, memory,
logging. Which can't be said about collations - they are used
only by high level things - SQL, Lua utf8, comparators.

Collations are built now as 'lib/coll' library.

The patch is not necessary for anything, but it is a right thing
to do, while some activity is happening there.