Corpora which store on-disk: permit skipping the generation of .lafl_lock files + duplicates with postfix

[addisoncrump]

In many scenarios, it is not necessary to create/check .lafl_lock nor to duplicate files (in the form <hash>-<number>). We should allow users to disable this functionality.

[bkrl]

What would be the best way to add this option to the API? Adding a new argument to the constructors would have to be a breaking change since Rust doesn't have default arguments yet, and adding new constructors doesn't seem to be ideal since there are already three for InMemoryOnDiskCorpus.

[domenukk]

What would be the best way to add this option to the API? Adding a new argument to the constructors would have to be a breaking change since Rust doesn't have default arguments yet, and adding new constructors doesn't seem to be ideal since there are already three for InMemoryOnDiskCorpus.

Sorry, missed that comment.
It might be useful to add a builder at some point, or simply make it a setting on the corpus after creation

[addisoncrump]

Implemented by #981.