Write fuzz harnesses to ready-to-use projects

[Shnatsel]

Fuzzing harness generator should operate in a manner similar to cargo fuzz init: it should create a subfolder in your project that would already have a Cargo.toml and have everything ready to go.

Since currently some harnesses may not be generate correctly and may not compile, we do not want to take the cargo-fuzz approach of "one project with many binaries", but rather generate multiple independently compiled projects so that if one fails to compile, the rest can still function.

[Shnatsel]

Right now fuzzing harnesses are not even written to individual files, they're just returned as strings. We probably want to write a layer on top of that.

[Shnatsel]

Sample Cargo.toml to generate: https://github.com/Shnatsel/image-png/blob/master/png-afl/Cargo.toml
The rest of that directory layout can be used as reference as well.

https://github.com/Eh2406/auto-fuzz-test/blob/master/src/crate_parse.rs provides facilities for parsing Cargo.toml of the crate to fuzz, but for now we can get away with just passing it on the command line - we'll need to add it anyway to support non-Cargo build systems eventually