Add deprecated_generic(), deprecated_class(), and deprecated_property()

[hadley]

Fixes#727. Fixes#730.

This PR adds a standard way to deprecate the pieces of an S7 API — generics, classes, and properties — so that old code keeps working but warns users to update.

API

All three helpers require when (the package version when the deprecation began) and share a method argument controlling how the deprecation is signalled: "base" (the default, a .Deprecated()-style warning attributed to the user's call), "lifecycle(warn)", or "lifecycle(stop)". The lifecycle options require lifecycle as a dependency of the deprecating package.

deprecated_generic()

# renamedsummarise:= deprecated_generic(new=summarize, when="1.1.0")
# retired with no replacementshout:= new_generic("x")
shout:= deprecated_generic(old=shout, when="2.0.0")

Calling the old name warns then delegates to the target (via call rewriting, so arguments pass through lazily and unmodified). method<-, method(), method_explain(), S7_methods(), and deferred external-generic registrations all silently unwrap to the target, so downstream packages keep installing and working. This also mitigates #729 for renames: a stale downstream registration finds the deprecated object and resolves through it.

deprecated_class()

Dog:= deprecated_class(new=Pet, when="2.0.0")

Calling the constructor warns then constructs an instance of new. In every other context (as_class() is the chokepoint: method signatures, parent, property classes, unions, convert(), S7_inherits(), new_external_class() resolution) the alias is silently treated as the replacement. The no-replacement form mirrors the generic: Cat := deprecated_class(old = Cat, when = "3.0.0").

deprecated_property()

Basket:= new_class(properties=list(
size=class_double,
deprecated_property("count", new="size", when="1.5.0")
))

Generalizes the getter/setter pattern from vignette("classes-objects") (which now uses it): reading or writing the old name warns and delegates to the replacement, and the generated default means the constructor only warns when the deprecated argument is actually supplied. With no new, the property stores data itself and warns on access (construction can't warn in that case, since S7 can't distinguish a user-supplied value from the default).

new_external_class() now resolves aliases (#727)

resolve_external_class_req() no longer requires the exported object's @name/@package to match the reference, so a class renamed with a plain alias (Foo <- Bar) or a deprecated_class() keeps working for downstream packages.