Added PartitionAt, supporting types, and tests

[awcullen]

First try...

[awcullen]

I see that Partition also allows you pass in a slice of Buffers with the result being one big logical buffer. The wrinkle is that the passed in buffers may be of varying lengths. In this case I believe WriteAt should not be able to lengthen any buffer except for the last. Let me fix this case and add a test.

[djherbis]

Hey, I'll review this when I get chance, but is there a reason you chose to continue down this route vs. just using something like io.MultiReader etc. mentioned in #7.

[djherbis]

First few comments:

• Don't commit debug.test
• Instead of adding the *At versions into the existing files, can we copy the existing files, add _at to them (except the test file), it'll make it easier for me to diff the files (I'm expecting the diff to basically just be in types except for ReadAt/WriteAt, and tests)
• Please reference the issue in the commit message Partition that implements BufferAt #7
• In your tests, please make sure you include the checkCap, runPerfectSeries tests as well

[janekolszak]

Hi,
Are you planning to merge?
Thanks

[djherbis]

Hey @janekolszak I'm editing the PR now, I've made some progress applying the changes I originally requested.

Once I've finished tidying it up I'll merge it.

[janekolszak]

Do you need any help?

[djherbis]

Sorry I was close to done but I went on vacation for a long weekend. Might have some time Monday

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[djherbis]

@janekolszak Merged it in, along with my changes and tests

[janekolszak]

Thank you so much! :)