Reproducible crate builds

[bk2204]

Describe the problem you are trying to solve
I'd like to provide a way for folks who receive my crate from crates.io to have confidence that it is exactly identical to the one built from the source.

Describe the solution you'd like
I'd like to have a reproducible crate build so that running crate package on two different systems (with the same version of cargo) produces bit-for-bit identical archives. I imagine that would look like this:

• Taking the date for every file in the archive from the SOURCE_DATE_EPOCH environment variable or the latest commit, if the working tree is unmodified and version controlled.
• Setting the user and group IDs to 0.
• Picking fixed values for device and inode numbers in the tar archive.

Possibly this could be controlled with a flag if having it as the default behavior isn't wanted.

Notes
More information about reproducible builds and why they're valuable can be found at https://reproducible-builds.org/. I'm happy to implement this if folks think it's a good idea.