[Roxygen-devel] Primitive S3/S4; NAMESPACE proof-of-concept

[Peter Danenberg]

I decided to defer the thorny issues of a unified @export/@import by
adding a gamut of tags:

    @export
    @exportClass
    @exportMethod
    @S3method
    @import
    @importFrom
    @importClassesFrom
    @importMethodsFrom

We can always do something more sophisticated in the NAMESPACE Roclet
by inferring types; by the naive solution allows for full control.

I've also provided for a S3-class description with:

    @S3class
    @slot
    @listObject
    @attributeObject
    @environmentObject

It requires, however, that we associate the Roxygen block with a
no-op; see `example-S3-mcpi.R'.

@returnType, @return, @author, @reference have been added; and the
parser has been changed to pass-through any unknown tags with a
warning. Later on, we can allow Roclet authors to register parser
callbacks.

See `sandbox/namespace.R' for a proof-of-concept NAMESPACE Roclet.