Allocating an instance of a class uses the allocator for the class. When
the class has no allocator set, Ruby looks for it in the super class
(see rb_get_alloc_func()).

It's uncommon for classes created from Ruby code to ever have an
allocator set, so it's common during the allocation process to search
all the way to BasicObject from the class with which the allocation is
being performed. This makes creating instances of classes that have
long ancestry chains more expensive than creating instances of classes
have that shorter ancestry chains.

Setting the allocator at class creation time removes the need to perform
a search for the alloctor during allocation.

This is a breaking change for C-extensions that assume that classes
created from Ruby code have no allocator set. Libraries that setup a
class hierarchy in Ruby code and then set the allocator on some parent
class, for example, can experience breakage. This seems like an unusual
use case and hopefully it is rare or non-existent in practice.

Rails has many classes that have upwards of 60 elements in the ancestry
chain and benchmark shows a significant improvement for allocating with
a class that includes 64 modules.

```
pre: ruby 3.0.0dev (2020-11-12T14:39:27Z master 63258664)
post: ruby 3.0.0dev (2020-11-12T20:15:30Z cut-allocator-lookup)

Comparison:
                  allocate_8_deep
                post:  10336985.6 i/s
                 pre:   8691873.1 i/s - 1.19x  slower

                 allocate_32_deep
                post:  10423181.2 i/s
                 pre:   6264879.1 i/s - 1.66x  slower

                 allocate_64_deep
                post:  10541851.2 i/s
                 pre:   4936321.5 i/s - 2.14x  slower

                allocate_128_deep
                post:  10451505.0 i/s
                 pre:   3031313.5 i/s - 3.45x  slower
```