Doctests for `parse-generics-shim`?

[ErichDonGubler]

This issue was opened so that the main generics issue (#18) can be closed and not lose a thread that may or may not be significant for future development around. The aforementioned thread is that as things stand, this project is using a modified version of source code from the parse-generics-shim crate. A summary of the transformation process is currently:

1. Strip out doctests, which break cargo test if not removed as-is.
2. Add a copyright for modified content, to fulfill the case where users of adhesion want an Apache license and need to include source with their binaries or libraries.

The core issue is that item 1 may or may not present risk to the codebase. It would be better to have tests run for the generics parsing modules themselves, but by the same token, it's not code that I plan to maintain myself. My personal feeling about this is that it's the job of the parse-generics-shim to make sure it's well-tested, and we simply consume what's available -- meaning that things like doctests aren't particularly valuable. It will probably be easiest to simply keep step with the parse-generics-shim crate and apply the process listed above whenever we need to update it as a dependency.