Remove class alloc check

[jhawthorn]

I want to remove the following runtime check in class_call_alloc_func, replacing it with a RUBY_ASSERT (so we would still have the check, but only on a debug build).

if (rb_obj_class(obj) != rb_class_real(klass)) {
    rb_raise(rb_eTypeError, "wrong instance allocation");
}

This checks that the value returned from the function registered with rb_define_alloc_func is of the correct class. When this was first introduced in 1fe40b7 (by Matz on 2001-10-03), allocation was done via user-defined Object#allocate, so it made sense to have a runtime check in release builds. Now that it's defined via rb_define_alloc_func in the C extension API, I don't think it's necessary.

The check is surprisingly expensive. Removing it makes Object.new about 10% faster. It allows the C compiler to optimize the call to the function pointer as a tail-call. Removing this also allows ZJIT/YJIT to call the function directly (ZJIT already does this by having a list of known safe allocation functions). The diff in the assembly from skipping this is substantial https://gist.github.com/jhawthorn/20834b654bec4ba0bafee729592d5156 (though much of that is rb_class_real which I also have plans to improve).

$ ips --ruby './miniruby_before' --ruby './miniruby' -e 'Object.new' --yjit
make: 'miniruby' is up to date.
ruby 4.1.0dev (2026-03-26T21:37:39Z master 174ace6af7) +YJIT +PRISM [x86_64-linux]
          Object.new:    86.046M i/s (± 1.1%, GC 24.6%)

ruby 4.1.0dev (2026-03-26T21:44:56Z remove_alloc_class.. e497b65a12) +YJIT +PRISM [x86_64-linux]
          Object.new:    96.527M i/s (± 0.4%, GC 20.3%)


Summary
  ruby ./miniruby ran
    1.12 ± 0.01 times faster than ruby ./miniruby_before

There's no way for users to ever have seen this check, other than by writing a misbehaving C extension, which returns objects with the wrong class.