Simplify send/send_mmsg API

[shutton]

Eliminate need to supply elided generic type to send/send_mmsg calls with non-default batch size.

[leshow]

This is kind of a subjective thing, but in general I try to avoid in-argument impl Trait in public APIs. The original syntax has the advantage of allowing you to turbofish and specify the argument type, which can be useful sometimes, and it is a breaking API change to switch it.

[leshow]

I suppose we already changed the API with the batch size. Anyway, I'm just going to leave it. I do appreciate the contributions though :)

[shutton]

Understood. I didn't like that, in order to tweak the useful generic (const), you had to go through the less-likely-to-be-specified generic (i.e., the turbofish always looks like ::<_, BATCH_SIZE>). But I can live with it.