Set up some fuzz test harnesses for the UntrustedRlp struct
and RlpStream's append_raw function.

Only attempt to compile fuzz tests using a nightly compiler

Wrap the entire source files for fuzz tests in the nightly
feature. Meaning tests will only be included in compilation
when using a nightly compiler.

Include entire source in config macro

Re-checked macro syntax, and changed to match correct syntax.

Included `nightly` feature for entire fuzz test files, excluding
their compilation during stable builds.

Some code setting rustc config lines were unnecessary and removed.
At the moment, only the `nightly` feature is enabled when running
fuzz tests with `cargo fuzz run <test>` and a nightly compiler.

For UntrustedRlp to be processed as a list, one needs to prepend
the data with the byte `0xc-` where `-` is the number of items
in the list. Currently using the length of the fuzz data stream
as the number of items in the UntrustedRlp list.

Added an algorithm `iter_recursive()` to iterate recursively over
the entire RLP structure. Also created a tool `gen_untrusted_corpus.rs`
for generating valid RLP structured data to make the fuzzing process
more efficient.

Separated writing the RLP stream to its own function, and build
the file path using PathBuf.

libFuzzer uses dictionaries of keywords to mutate/generate new strings.
I added a six-entry dictionary of sample RLP streams found in the
Ethereum documentation. Also removed recursive iteration from the fuzz
test, replacing it with multiple calls to `as_val()` decoding the
`UntrustedRlp`'s base value as various types. HUGE boost in code
coverage with only a relatively small cost to executions/second.

…zz-tests