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(insert_from_string_1, insert_from_buffer_1): Use it.
(copy_text): New function.
(insert_from_buffer_1): Call copy_text to convert between
single-byte and multibyte text.
(insert_from_string_1): Likewise.
(insert_1_both): New function.
(insert_from_string): New args POS_BYTE, LENGTH.
(insert_from_string_before_markers):
(insert_from_string_1): New arg POS_BYTE.  Swap args NCHARS, NBYTES.
Don't fail to use POS_BYTE!
(insert_from_buffer_1): Get both inschars and insbytes from NEW.