Debug-mode detailed output

[lukacan]

I would say currently the best possible option to debug the fuzzed instructions sequence is to use msg!() macro inside the solana program.

This PR makes it easier to examine data and accounts passed into instructions. Here are the improvements:

• It defaults to deriving the Debug trait for Accounts and Data structs generated by the Fuzzer.
• The DisplayIx macro is extended to also display struct contents.
• In relation to the above points, the DisplayIx macro is further extended to derive a "to_context_string()" function. This is used to obtain the instruction name since the std::fmt::Display implementation now includes the struct contents, meaning to_string() would also fetch the contents - this method is further used within the FuzzTestExecutor to obtain Error Context.
• (This may be overkill) A simple "show_account" macro is introduced, which implements pretty-print for the input account.
  • This may be used by the user for example within the check() function. The macro contains requirements inside rustdocs to derive the Debug trait for the Account Struct If the user wants to display the Account`s contents.

When we look at the data from instruction sequences, the amount of information in debug mode can be too noisy. I thought about using an additional conditional compilation to manage this. This means we could use the RUSTFLAGS environment variable to give extra instructions to the rustc compiler (used by the honggfuzz). But, doing this would mean that all the code that was compiled before would need to be compiled again, and this could take a lot of time. So, for now, I've decided to just use the fuzzing_debug flag, which is automatically turned on by the honggfuzz.

[Ikrk]

lgtm