Update to Rust 2018 Edition

[lrazovic]

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This PR address some general updates:

• Upgrade rand dependency from 0.7 to 0.8
• Use the Rust 2018 syntax, and update the relative documentation (removed external crate from code and doc)
• Update the fuzzer code to Rust 2018 syntax and update libfuzzer-sys to the latest public version.
• Fix two clippy warnings:
  • redundant field names in struct initialization
  • #[inline] is ignored on function prototypes