Extend SU.collatedSort for complex table sorting

[Omikhleia]

I noticed when working on #2082 : Our SU.collatedSort() works on list of strings only:

local names = {"Charlie", "Bob", "Alice"}
SU.collatedSort(names)
--  gives: {"Alice", "Bob", "Charlie"}

My own bad at the time (#1632)... but we can't use it to sort structured (key-value) tables with a comparison callback, as one would do with table.sort().

Of course there's a workaround, but it's a bit annoying (comparing for string equality, then performing a second ICU comparison for ordering)

So this PR is a proposal for supporting it, as:

-- Sort by age then name
local namesAndAges = { {name="Charlie", age=25}, {name="Bob", age=30}, {name="Alice", age=25} }
SU.collatedSort(namesAndAges, nil, function (a, b, stringCompare)
  if a.age < b.age then return true end
  if a.age > b.age then return false end
  return stringCompare(a.name, b.name) < 0
end)
-- gives { {name="Alice", age=25}, {name="Charlie", age=25}, {name="Bob", age=30} }

-- Sort by name then year
local namesAndYears = { {name="Alice", year=2005}, {name="Charlie", year=1995}, {name="Bob", year=1990}, {name="Alice", year=1995} }
SU.collatedSort(namesAndYears, nil, function (a, b, stringCompare)
  local nameCompare = stringCompare(a.name, b.name)
  if nameCompare < 0 then return true end
  if nameCompare > 0 then return false end
  return a.year < b.year
end)
-- gives { {name="Alice", year=1995}, { name = "Alice", year = 2005 }, {name="Bob", year=1990}, {name="Charlie", year=1995} }

In other terms:

• Be less clever in the C code and just return -1, 0, 1 for (less, equal, greater)
• Use a closure to pass ICU-enabled string comparison to the user callback.

[Omikhleia]

Oops sorry for the multiple push force... Was on wrongly rebased branch :)