general fuzz seeder

[zeroXbrock]

• New traits Seeder and SeedValue allow devs to implement their own number-generators to use
• removed Seeder type bounds from Spammer and Generator
  • Not all generators need seeders, so it's up to the writer of impl Generator for YourStructHere to manage their own Seeder (if they need one).
• Spammer's implementation now only relies on an impl Generator.

A prime example of a Generator implementation is here -- this is used to generate txs from a config file.