1.1.1

CHANGES:

• Add of_fmt to compose with existing pretty printers written in Format
  (#1).

• Use a tail-recursive List.map to fix a stack overflow issue (#3,
  @emillon)

• Add Pp.custom_break (#4, @gpetiot)

• Add Ast sub-module to expose a stable representation for
  serialization, allowing to do the rendering in another process (#6)