Reduces crafting acts needed to make long rope

In partial response to: https://github.com/CleverRaven/Cataclysm-DDA/issues/8895

Makes long rope and short rope craftable from long and short string. Makes long string craftable from thread/sinew/plant fibre.

Keeps the exchange rate (although it could be made lossy, as Kevin said here: http://smf.cataclysmdda.com/index.php?topic=3088.msg44407#msg44407 - please comment), but makes crafting long rope less of a bother (going from 9000 thread to 1 long rope is currently 216 acts of crafting, with the "fix" it will be 31 acts (craft 30 long strings and craft 1 rope from them) - balanced the lessened hustle with +50% crafting time for both types of rope).