Consider migrating to only use 7-zip not p7zip

[ronaldtse]

7-zip now offers an SDK and supports all major platforms we want to support.

• https://www.7-zip.org/sdk.html

There is an existing Python binding for the SDK here:

• https://github.com/fancycode/pylzma

Perhaps we can drop p7zip and instead, migrate the API of this gem to the API used by the SDK. This will give us confidence that the gem will work in the future with future 7-zip versions. We also won't need to keep the binary DLL files here.

[ronaldtse]

Actually, the SDK only supports LZMA.

The full source of 7-zip can be found here (a GitHub mirror of the original):

• https://github.com/mcmilk/7-Zip