Restructure the source files

[ephiepark]

This pull request reorganizes codes into different files.
This changes have the following side effect:

• functions like ZSTD_noCompressLiterals, ZSTD_compressRleLiteralsBlock, ZSTD_selectEncodingType, ZSTD_buildCTable, ZSTD_encodeSequences are no longer static functions.

[terrelln]

This is great! zstd_compress.c is gigantic

[Cyan4973]

The change itself looks good to me.

Now, the main issue is around build scripts.
Some of them will have to be updated, because they can't just "grab the directory content", as Makefile does (or are not designed this way, and maybe they should be updated to do so).

This includes :

• cmake
• Visual Studio
• meson

For the PR to pass, all build systems must be updated.
This is a major source of slow down.

At this stage, a quickfix is good enough.
though if you can find a way to make some of these build systems a bit more "generic",
this would be a great gain too.

[ephiepark]

@Cyan4973 Thanks for the comment! I was confused about what was causing the issue. Let me look into them.

[ephiepark]

This pull request had one failure on Travis CI on (Trusty (gcc-8 + ASan + UBSan + Fuzz Test)). The cause of the failure is independent from this pull request. It's caused because gcc8 is not properly suppressing signed-integer-overflow (pointer-overflow) for ubsan. I have created an issue for it (#1686).

[Cyan4973]

Thanks @ephiepark , this new GLOB is much better for file layout flexibility

[Cyan4973]

Thanks @ephiepark ,
this patch is good to go.

As we are close to a new release, we'll wait a bit for the "freeze" snapshot, and merge this patch right after that.

[Cyan4973]

Release completed,
we can merge.