Mitigate generics

[cgwalters]

See bootc-dev/bootc#1314 (comment)

In live chat Allison was thinking we could at least pick a default hash value; that would very clearly help in reducing typing. Maybe we could make it a repository build time option?

But I still have a medium-strong opinion (as came up in the original PR #105 ) that it'd be better to just do dynamic dispatch in general.

There's variants here; e.g. we might keep generics for e.g. pub struct FileSystem.

I dunno though, I meant we already heap allocate for each filename, extended attribute and inline content, are we really winning by inlining RegularFile::External::ObjectId?

[cgwalters]

#167 touches on this slightly