copy and length coercion

I notced that COPY FROM does not invoke the length coercion function
before calling heap_insert(). This leads sometimes bad things such as
incorrectly truncated mutibyte strings. My idea is finding an
appropreate function like currently
coerce_type_typmod(parser/parse_coerce.c) does, and calling it before
heap_insert()

Comments?
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Tatsuo Ishii