rpmlib: RPM database support

[tony-iqlusion]

This issue tracks support for interacting with the RPM database. Presently no support is available.

Some reference information on the API is available here, courtesy the Fedora RPM Guide:

C API:

• https://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch15s04.html
• http://ftp.rpm.org/api/4.11.1/group__rpmts.html#gab4b02a71a720d4df382383cfef2b193b

Python API:

• https://docs.fedoraproject.org/ro/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch16s03.html

The Python API informs us what the first hurdle will be:

Just about every Python RPM script needs a transaction set.

~~~So, the first step is figuring out how to resolve the bindgen test failure.~~~

~~~If you are interested in helping this issue move forward, the main place to start is investigating #11~~~

#11 is fixed, and #19 added initial support for transaction sets. This is ready to begin working on, if anyone is interested in contributing.

Mapping the RPM database APIs to idiomatic Rust looks fairly straightforward. The main API to the database, beyond a transaction set, is a transaction set iterator, and we all know how much Rust loves iterators. The docs also give clear instructions about the lifetime of memory, including the appropriate times where transaction-related objects should be freed (or don't need to be freed).

[tony-iqlusion]

Initial support for reading/querying the database was added in #24, however there's still quite a few more features that would be nice to add. I will open separate issues for those.