Add page writer

[sadikovi]

This PR adds:

• PageWriter interface and serialised implementation.
• CompressedPage in addition to Page. Compressed page is a wrapper that allows us to store compressed buffer + uncompressed length. Page, when created, is always assumed to have uncompressed buffer. This is not the case for compressed page - internally we store compressed data.
• Changed SerializedPageReader to take T: Read instead of file source. This allows me to test a page roundtrip.
• Added tests.

[sadikovi]

@sunchao I implemented page writer. Let me know if I need to add more tests, I feel like the currently added tests may not cover all of the cases. Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 560

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 95.442%

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Files with Coverage Reduction	New Missed Lines	%
encodings/encoding.rs	1	94.8%
column/page.rs	4	96.3%
<!--	Total:	5		-->

Totals
Change from base Build 558:	0.05%
Covered Lines:	11348
Relevant Lines:	11890

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💛 - Coveralls

[sadikovi]

@sunchao can you have a look again? I addressed your comments and added more docs describing each method.

I was thinking if PageWriter trait needs an improvement; it might be a bit difficult to reason about the API without column writer. Let me know what you think.

Thanks!

[sadikovi]

Yes, you are right, it does. We should be able to refactor both when working on column chunk writer.

[sunchao]

Merged. Thanks @sadikovi !

[sadikovi]

Thanks @sunchao!