Fix catastrophic performance

[Dreamsorcerer]

Fixes #819.

Without patching invoke, no matter how much hardware is thrown at it, it will never exceed a throughput of 100 B/s (yes, bytes). This is a multi-pronged approach to fix the problem.

• When the output is not an actual terminal, use the chunk size instead of falling back to 1 byte chunks.
• If we received data, don't sleep for 10ms between chunks when there's probably more data to be processed.
• Increase the default chunk size to 64 KiB, which better matches other typical default sizes.
• Add a chunk size parameter to allow the user to use larger chunk sizes.

The first 3 points improve performance from 100 B/s to atleast comfortably over 10 MiB/s. Using a custom 2 MiB chunk size via point 4, I was able to get an extra ~6% performance on top of that.

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Combined with #915, this will finally allow me to stop patching invoke and just use it as installed library.