Only parse as much of files as needed

[cmeister2]

In fez I added an RpmPkgReader class (https://github.com/cmeister2/fez/blob/main/src/rpm/package.rs#L233) which allowed for "on-demand" reading of RPM files - if you only wanted to read the metadata, you could, without having to download the whole package first.

I tried for a while to come up with a sensible design for this, with several issues:

• the mutability of the Read object needs to be taken into account
• working out what data has been read and what data has not been read, and caching the data that has been read, made for a confusing time. I didn't find a wonderfully smooth solution here.

It'd be nice if it didn't have to be a separate class, if possible.

[dralley]

RPM headers usually aren't that big, so I'm thinking a convenient use case might be to just read the header into a &[u8] and parse it from there directly, which solves both problems - no need for caching because it's already in memory, and the data is immutable.