Remove optional gen parameter from bytes_size and string_size generators

[jmid]

As part of The Great Renaming in 0.90 we kicked a couple of cans further down the road, regarding optional gen parameters to the bytes_size and string_size generators.

From #371 on bytes:

With bytes_size_of added there's no need for the optional ?gen parameter in all 3 variants of bytes_size.
I'll leave to the 1.0.0 release to remove it.

and from #372 on string:

Like for bytes in #371, this postpones the removal of (now redundant) optional ?gen parameters to string_size to the future 1.0.0 release.

To move forward with the 1.0.0 release, this PR thus keeps the promise and removes the redundant ?gen parameter.
This is an API breaking change, which is precisely why it was postponed to the 1.0.0 release, for semantic-versioning-harmony 🌞

Any usage, e.g., bytes_size ~gen size can be easily rewritten to bytes_size_of size gen.