Move these macros from include/ruby/backward.h to
include/ruby/internal/attr/deprecated.h, alongside the other similar
macros.

include/ruby/internal/intern/vm.h cannot currently use them because
include/ruby/backward.h is included too late.

This reverts commits for [Feature #18589]:
* 8008fb7352abc6fba433b99bf20763cf0d4adb38
  "Update formatting per feedback"
* 8f6eaca2e19828e92ecdb28b0fe693d606a03f96
  "Delete ID from constant cache table if it becomes empty on ISEQ free"
* 629908586b4bead1103267652f8b96b1083573a8
  "Finer-grained inline constant cache invalidation"

MSWin builds on AppVeyor have been crashing since the merger.

Current behavior - caches depend on a global counter. All constant mutations cause caches to be invalidated.

```ruby
class A
  B = 1
end

def foo
  A::B # inline cache depends on global counter
end

foo # populate inline cache
foo # hit inline cache

C = 1 # global counter increments, all caches are invalidated

foo # misses inline cache due to `C = 1`
```

Proposed behavior - caches depend on name components. Only constant mutations with corresponding names will invalidate the cache.

```ruby
class A
  B = 1
end

def foo
  A::B # inline cache depends constants named "A" and "B"
end

foo # populate inline cache
foo # hit inline cache

C = 1 # caches that depend on the name "C" are invalidated

foo # hits inline cache because IC only depends on "A" and "B"
```

Examples of breaking the new cache:

```ruby
module C
  # Breaks `foo` cache because "A" constant is set and the cache in foo depends
  # on "A" and "B"
  class A; end
end

B = 1
```

We expect the new cache scheme to be invalidated less often because names aren't frequently reused. With the cache being invalidated less, we can rely on its stability more to keep our constant references fast and reduce the need to throw away generated code in YJIT.

In the past, many internal functions are declared in intern.h
under include/ruby directory, because there were no headers for
internal use.

There is nothing interesting here. [ci skip]

Get rid of duplicate attributes, which may be warned or ignored
except for the first.

* DECLARE_DEPRECATED_FEATURE with RBIMPL_ATTR_DEPRECATED_SINCE
* DECLARE_DEPRECATED_INTERNAL_FEATURE with RBIMPL_ATTR_INTERNAL
And moved function declarations outside both.

Commit 7aab062ef3772c7e8e50fc872a1647918c76dbba says:

> ruby_show_version() will no longer exits the process, if
> RUBY_SHOW_COPYRIGHT_TO_DIE is set to 0.  This will be the default in
> the future.

3.0 is a good timing for that "future".