…ng `Clone + Send`.

* Added `Fuzzer::load_seed_input` to allow additional or different handling
  of seed inputs, e.g., parsing seed files that are then stored in a
  different format internally and in the corpus (e.g., parsing source
  code into an AST and then doing AST mutations).
* A fuzzer is now constructed with `FUZZER::new` once and then cloned
  for every core. As a consequence fuzzers are now `Clone + Send`. This
  allows for several patterns:
    * Custom shared state across all cores, e.g., for custom metadata.
    * Performing costly initialization only once (e.g., parsing a system call
      definition file).

… all commands.