More bounds checking against integer overflows

• Fix arithmetic overflow in Cstruct.lenv and copyv (#159 by @yallop)
• Reject negative destination offsets in blit (#160 by @yallop)
• Add AFL fuzz tests using Crowbar, which independently discovered
  #160 and also an overflow in of_bigarray and sub, now bith
  fixed (#164 by @talex5)
• Improve performance of several allocation functions by eliminating an
  unnecessary buffer zero step (#158 by @hannesm)
• Compile the source tree with stricter flags, including dead variable
  detection and deprecation warnings (#157 by @samoht)
• Bump the required minimum OCaml version up to 4.03.0 (due to #157).