Add script to generate fuzz coverage

[Prabhat1308]

This script generates the fuzz coverage in a html file when run .

Considerations

1. The script assumes a corpus directory in fuzz directory.
2. Also assumes the corpus for a particular target to be under the name of the target itself. for eg - corpus for base32_target.rs would be under corpus/base32_target.
3. The corpus I worked with was generated using libFuzzer it may/may not work with corpus generated from other fuzzers.

How to run

# from the root directory
chmod +x contrib/generate_fuzz_coverage.sh
contrib/generate_fuzz_coverage.sh