Add deny.toml

[emhane]

solana-sha256-hasher has been locked at solana-define-syscall version 4.0

Add deny.toml and run deny check in CI to catch multiple versions

[bans]
multiple-versions = "warn"

[joncinque]

Sorry, I don't think the problem is clear -- is there something wrong with multiple versions of solana-define-syscall? For that crate in particular, since it's just defining functions, we've judged that there isn't a problem with multiple versions in a build.