Add support for fuzzing

[Shnatsel]

• Bypass crc32 and adler32 checks via conditional compilation when lodepng is compiled with fuzzing instrumentation. This lets random input from fuzzers actually reach interesting decoding code instead of being rejected early on due to checksum mismatch.
• Add fuzzing harness for cargo-fuzz that was used to discover #28

If you do not wish to have "fuzz" folder within the project, I can put it in https://github.com/rust-fuzz/targets repository instead.

[kornelski]

Thank you.

[kornelski]

The fix for it was trivial — deletion of one line: 37a7bcc

And the leak existed there only because this crate keeps ABI compatibility with its C version, so it has to malloc strings. I'll probably drop the ABI compatibility, because Vec<String> would make things so much simpler.

[Shnatsel]

I feel C ABI compatibility is a major selling point of this crate compared to something Rust-only, such as png crate.