Fuzzer

[martinling]

This PR adds a fuzzer which tries to break the decoder under the following rules:

• The decoder input may be any number of timestamped raw packets.
• No constraints are placed on the number, length or content of packets.
• To avoid uninteresting overflows/underflows in the timestamp compression, timestamps are constrained to start with a 32-bit value and increment in 32-bit deltas.

This approach very quickly identified some failures, which are fixed in the following commits.

The fuzzing is implemented using the libfuzzer_sys crate. We can't use cargo-fuzz in this case because we are testing private internal APIs. Instead, the fuzzer is implemented as a standalone example program which imports required modules directly.

The fuzzer can be run with cargo run --features fuzzer --example fuzzer, but for best results it is necessary to use the nightly compiler and build with:

cargo +nightly rustc --features fuzzer --example fuzzer -- \
     -C passes='sancov-module' \
     -C llvm-args='-sanitizer-coverage-level=3' \
     -C llvm-args='-sanitizer-coverage-inline-8bit-counters' \
     -Z sanitizer=address

and then run with:

target/debug/examples/fuzzer