Don't rely on libfuzzer-provided entrypoint

[nagisa]

So, this can definitely work without changes to libfuzzer itself, as it
is written today. There are a couple pieces to it:

1. Don't compile FuzzerMain.cpp at all;
2. Use LLVMFuzzerRunDriver defined in FuzzerDriver.cpp to kick off
   the fuzzing process in the macro.

I think with those and something like the inventory crate we also open
ourselves to having interface that's more like libtest.

That said, LLVMFuzzerRunDriver requires argc and argv, which at
that point requires one to manually convert them back into C layout from
std::env::args_os.

I also don't believe this change is meaningful as is, without an
otherwise major rework of the libfuzzer API. It doesn't achieve
anything much as it is, and only serves to complicate the implementation
of libfuzzer crate itself.

cc #46

[nagisa]

(it doesn't build right now because there are some changes wrt visibility within macro, which I didn't consider a good expenditure of time, given the other observations)

[nagisa]

Returning to this, I think it might be worthwhile to do this. Among other things that this can enable is e.g. an ability to specify certain fuzzing-related parameters within the test case itself. So e.g. they could specify sensible defaults for the fuzz target from within the source.

[jameysharp]

I think you can make this available without breaking the existing libfuzzer crate API by splitting the crate into two parts.

One crate links in the "no-main" version of libFuzzer (which is apparently a standard build configuration these days). That crate should just export bindings to functions like LLVMFuzzerRunDriver.

The other crate exposes the current libfuzzer-sys API by defining main. Maybe that can be a C-compatible main (#[no_mangle] etc) so you don't have to convert back from std::env::args_os?

I'm not actually sure how that would work. I had C-compatible main functions working in Corrode-generated Rust modules five years ago, but then I was doing the final link with gcc, so probably everything's quite different.

[addisoncrump]

Alternatively, you can apply the strategy of #106 and feature-gate the linkage.