patch 9.1.1137: ins_str() is inefficient by calling STRLEN()

patch 9.1.1137: ins_str() is inefficient by calling STRLEN()

Problem:  ins_str() is inefficient by calling STRLLEN()
Solution: refactor ins_str() to take a length argument
          and let all callers provide the correct length
          when calling ins_str() (John Marriott)

closes: #16711

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>