Ability to save PSD's

[mexicantexan]

I'm happy to take this up! Just point me in the right direction.

Right now the psd crate only supports deserializing a PSD from bytes. It does not currently support writing PSD bytes to a file. I'd be happy to help guide anyone that is interested in contributing support for this.

Originally posted by @chinedufn in #51 (comment)

[chinedufn]

I would recommend starting with:

• come up with a few very basic writing scenarios. For instance, "saving an empty file", "saving a 1x1 file"
• create a mod write_bytes or something like that
• add tests for the basic scenarios
• get those tests passing
• submit a PR

We want to be able to round-trip PSDs such that if we write a PSD, then read it, it equals the exact same in memory Rust PSD that we wrote.
So, our test suite should test that.

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In short, step 1 here is just getting some tests written for simple cases.

Then from there we can continue to test more and more cases.

Please feel free to continue asking questions.