Make S7 generics and methods traceable

[hadley]

Fixes #584.

Allow trace() and untrace() to work with S7 generics and methods, including tracing both during multiple dispatch. For example, trace("my_class", browser, where = my_generic@methods) sets a breakpoint in the method for my_class. Traced functions retain their S7 properties and representation metadata, so method lookup, property access, and printing continue to work. untrace() restores the original function.

Internally, this declares the S7 generic and method properties as S4 slots, exposes the .Data slot needed by R's tracing initializer, and recognizes traced generics during native dispatch and method lookup. It moves methods from Suggests to Imports so registration can happen directly during package loading.

R versions before 4.7 use a compatibility initializer that preserves S7 metadata. On R 4.7 and later, S7 uses R's initializer, incorporating the upstream fixes discussed in r-devel/r-svn#262. Those fixes complement the slot declarations and dispatch changes here.

Local measurements on macOS with R 4.6.1, comparing this PR with main (245aaf46):

• Untraced single and multiple dispatch stayed at about 1.8 µs and 2.6 µs, respectively, before and after the change (1.0× faster, within measurement noise), with no measured allocation.
• Loading S7 increased from about 11 ms to 45 ms with methods already loaded (main loads 4.3× faster). This adds about 35 ms once per fresh session for the S4 registrations.
• With an empty tracer attached, single dispatch increased from 1.8 µs to 3.1 µs for a traced method or 3.4 µs for a traced generic (untraced dispatch is 1.7× or 1.9× faster). These costs apply only while tracing is active.

The dispatch and tracing measurements can be reproduced with bench/method-trace.R against separately installed builds.

[hadley]

Could you give us a brief (AI generated) summary of why all this additional machinery is necessary?

[t-kalinowski]

trace() replaces an S7 function with an S4 wrapper. The additional code keeps that wrapper usable as an S7 generic or method:

• Preserve S7 metadata. Register the properties and internal class/version attributes as S4 slots, following [Michael’s suggestion](Make trace() work for functions whose S3 class has multiple strings r-devel/r-svn#262 (comment)). This lets R’s tracing machinery preserve them. The .Data change lets R read the underlying function through S7’s @.
• Support method lookup as well as dispatch. The original PR unwrapped traced generics when calling them, but method() needed the same treatment. R’s C-level inheritance check doesn’t recognize the wrapper’s S4 inheritance.
• Support older R versions. The custom initializer handles the class-vector and coercion bugs before R 4.7. On R 4.7+, we use R’s initializer; the slot declarations and dispatch changes are still needed.

Importing methods lets us register everything directly, without deferred-loading hooks. Most of the added lines are regression tests and benchmarks.

[hadley]

I can't approve my own PR but I think this is good to go.