`save` API is a bit unintuitive

[JeffBezanson]

Really nice package! Just taking a close look at the API.

save is a bit odd to me since it doesn't actually save anything unless the function you pass also calls save. Some possible ideas:

• Make save(path(file), file.value) the default.
• Call it foreach instead.
• Something else?

[shashi]

Call it foreach instead.

I like this the most!

[shashi]

But then save also creates the right paths with mkpath before allowing you to write to that path...