(ocaml) Rename piqilib/piqi_piqi.ml to piqilib/piqi_lang_piqi.ml
(ocaml) Rename piqilib/piqi_piqi.ml to piqilib/piqi_lang_piqi.ml

The new name follows the purpose of the module and helps to avoid any possible
interference between stock piqi_piqi.ml or whatever an application decides it
should be.

alavrik/piqi-ocaml