Ppmd-cffi

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Changed
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* Change mem parameter in MB to mem_size in bytes.
* Change default branch name to main.

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Added
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* Add documennts.

Changed
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* API change
- Base API: PpmdEncoder PpmdDecoder class
- Simple API: PpmdBufferEncoder, PpmdBufferDecoder class

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* Change decode API to IOBase.readinto(b)

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* Change directory structure.

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* Change API to buffer protocol.

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* Release automation and support manylinux binary.