Merge rust-bitcoin#547: More fuzztest cleanups

a6405f8 fuzz: replace README.md (Andrew Poelstra)
7b04860 fuzz: delete old ./travis-fuzz.sh (Andrew Poelstra)
8e8dcb4 fuzz: remove afl support, run clippy and fmt on fuzz targets (Andrew Poelstra)
14ceccd fuzz: remove existing CI; run fuzz/generate-files.sh to make new one (Andrew Poelstra)
6fea17f fuzz: copy *.sh files from rust-bitcoin; tweak generate-files.sh (Andrew Poelstra)

Pull request description:

  Followup to rust-bitcoin#538. Copies the new fuzzing scripts from rust-bitcoin and cleans some things up.

  Confirmed that I can run all these fuzztests in my local nix-based CI.

ACKs for top commit:
  sanket1729:
    utACK a6405f8

Tree-SHA512: f19fa7a62e244aaebf3ac0a0bcfe1686392eca3679d71edd8d6504888bf1a5004a7527fa46e8ee6296be4a27dfc20408a8ea258569f6337a73f41118d751a451