rename module ChrName to Chr
Chr: make chr prefix optional as documented

Closes #105.
Chr: fix doc to reflect implementation
Chr: remove old internal of_string function

Should have been removed in 1700577b6aa58a02022f58b01fe7b93bc76e527c.
Chr: pure whitesapce fix
Chr: change function names and reorganize internal code
Chr: provide exceptionless version of to_roman

This follows Biocaml's convention for error handling.
Chr: remove unnecessary use of external modules

Pos was not being used at all. Msg was used trivially. Better to remove
this dependency.