Add a user-friendly way to build trees about gitoxide

[Byron]

I'd definitely want to have something like TreeUpdateBuilder at some point as well, if only for completeness. I am saying this as I think that an index.to_tree() may happen before this issue is tackled as git add and friends will work with an index. Using an index has further advantages as it can re-use trees that haven't changed thanks to its TREE extension Of course, if every tree is fully generated, or they are small enough, there probably wouldn't be much of a benefit.

In any case, I will keep it in mind and see that I can make it happen maybe even before the cargo integration is done (cargo doesn't seem to be building trees (yet)).