fix alignment issues

[nil-vr]

• A test is added for seeking backwards into the current page. This always worked because the page is flushed before seeking, effectively making the current page a previous page.
• A test is added for performing a write that crosses from one written page into another written page, and the implementation is fixed such that the second written page is not zeroed. This requires that the output stream is seekable when writing more than one page, but writing e57 files already requires that the output stream be seekable so this shouldn't be a problem.
• The PagedWriter no longer allocates any vectors.
• The writer is aligned after writing images. I'm pretty sure the C++ implementation expects this.

[cry-inc]

Thanks a ton for this PR! I will try to do a detailed review + merge + bugfix release in the next days. I am just not yet sure when exactly I will find the time to do so.

[cry-inc]

Today I merged your changes and released a new version of the crate. I added some additional changes as well. The important ones are:

• Slightly refactored and extended the tests for the paged writer to cover more scenarios
• Align writer to 4-byte boundary after every blob instead of after an image that could already contain multiple blobs

Thanks again for reporting this and also submitting a PR with fixes!