Implement cpio in Elixir to remove dependency

[fhunleth]

The use of cpio in bakeware is pretty simplistic and shelling out was done for expediency during the competition. Several people have reported issues with shelling out and it seems simpler just to avoid the issue.

Here's how we use cpio:

For each file and directory in the release do the following:
  Run stat to get the file/directory's type, permissions, etc.
  Write the CPIO header
  If a file, write its contents
When done, write the CPIO trailer

The CPIO header is a fixed length text string. You can see it at https://github.com/bake-bake-bake/bakeware/blob/main/src/cpio.c#L74. The hardest part, I think, is getting the padding correct. Many of the fields are ignored. Forcing those to 0 seems better than trying to get them right for deterministic builds and better compression.