Include charset.h and coding.h.

Include charset.h and coding.h.
(READ_BUF_SIZE): New macro.
(Finsert_file_contents): Refer to a coding system in the docstring.
Perform character code conversion of a text read in.
(Fwrite_region): Refer to a coding system in the docstring.
Setup a coding system for character code conversion.
Pass a new arg `pre_write_conversion' (Lisp function) to
build_annotations.
Pass a new arg `coding' to a_write.
(build_annotations): Handle the new arg.
(a_write): Handle the new arg `coding' by passing it to e_write.
(WRITE_BUF_SIZE): New macro.
(e_write): Perform character code conversion of a text to write
out according to the new arg `coding'.