Add export to Lean

[fblanqui]

• add the options '-o stt_lean' (tested) and '-o raw_lean' (not tested)
• add the option '--arities'

TODO:

• factorize the beginning of coq.ml and lean.ml into stt.ml
• update doc
• in Lean.print: prefix namespace by root_path
• check identifiers validity (following coq export: check that identifiers are valid in rocq #1397)