Flag load_all environments

[hadley]

Package environments created with load_all should contain flag object (.devtools?) that makes it possible to tell if a package was loaded with library or load_all, and a function to make this easy.

[wch]

I think a good way to do this is to add an object (either a list or an environment) in the package's namespace called .__DEVTOOLS__, which would hold the metadata. This is similar to how metadata about the namespace is stored, in an environment called .__NAMESPACE__.

It could also hold the status of .onLoad, and .onAttach, which are presently stored in variables in the namespace.

[wch]

Implemented in #126.