This change is an attempt to address the behavior found at
google/oss-fuzz#8389 where two distinct bugs were accidentally
deduplicated into the same bug report. One of the reasons for this is
that the stack traces between the two bugs were almost the same with
only very minor differences. My hope is that by forcing a unique stack
frame per fuzzer this will be less likely since there is guaranteed to
be at least one stack frame per fuzz target which is unique with this
change.

While I was here I wrapped up the generated function by the
`fuzz_target!` macro in a `const _: () = { ... }` to avoid adding this
new `run` function in to the normal module's namespace and accidentally
causing name collisions (e.g. if fuzz targets already have functions
named `run`)

Force at least one unique stack frame per fuzzer

This allows the fuzz target to indiciate whether an input was useful
for the fuzz testing by returning Corpus::Keep or Corpus::Reject.
Backwards compatibility is preserved by coercing the unit type () to
Corpus::Keep.

This maps to 0 (Keep) and -1 (Reject) in the libFuzzer API:
https://llvm.org/docs/LibFuzzer.html#rejecting-unwanted-inputs

Docs: make it explicit that we're ignoring the return value of the
function under test. Add comments from libfuzzer explaining why one
might want to keep inputs out of the corpus.

Convert From<Corpus> to i32 to a pub fn to_libfuzzer_code() that is
impl on Corpus to avoid accidental conversion.

Fix doc tests.

Enable keep/reject inputs from the corpus