Clean up some pa_llist remanents

A little bit of pa_llist was found in myocamlbuild and testsuite/_tags
Fix #363: generalize Set.map's type, add op_map

op_map covers the special case that the mapping function is order preserving and the tree doesn't need to be rebuilt.  Otherwise map rebuilds the whole tree
BatText: remove deprecated Invalid_rope exception
Merge pull request #365 from kerneis/remove-invalid-rope

BatText: remove deprecated Invalid_rope exception
batOption: addition of default_delayed and map_default_delayed.

  val default_delayed : (unit -> 'a) -> 'a option -> 'a
  val map_default_delayed : ('a -> 'b) -> (unit -> 'b) -> 'a option -> 'b
Fix #368: Enum.concat had wrong clone implementation
Fix #367: Add Set.find operation to match stdlib
Reformat all source code in src/ using ocp-indent; whitespace only changes
Fix bug in set.find order
Expose BatSet.find, add tests
BatText: add test for explode and implode
Fix list order in BatText.explode

The list of characters was output in reversed order.
fix BatFloat.round corner case on predecessor of 0.5
BatBitSet: Fix last element access
Merge pull request #369 from zakki/master

Fix batBitSet
New trivial function: List.singleton and Array.singleton

for completeness
And the compulsory tests
s/consisting in/consisting of/g in docstrings
Merge branch 'singleton'
List.unfold