[ruby-core:101913] [Ruby master Bug#17510] enable constant cache on Ractors

Issue #17510 has been reported by ko1 (Koichi Sasada).

----------------------------------------
Bug #17510: enable constant cache on Ractors
https://bugs.ruby-lang.org/issues/17510

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
constant cache `IC` is accessed by non-atomic manner and there are
thread-safety issues, so Ruby 3.0 disables to use const cache on
non-main ractors.

To enable the constant cache, we need to make `IC` access atomically,
and this is a patch: https://github.com/ruby/ruby/pull/4022

This patch enables it by introducing imemo_constcache and allocates
it by every re-fill of const cache like imemo_callcache.

Benchmark:

```ruby
Warning[:experimental] = false

def tarai(x, y, z) = Object && # useless constant reference
  x <= y ? y : tarai(tarai(x-1, y, z),
                     tarai(y-1, z, x),
                     tarai(z-1, x, y))
def task = tarai(12, 6, 0)
#
require 'benchmark'

Benchmark.bm{|x|
  x.report('seq'){ 4.times{ task } }
  x.report('par'){ 4.times.map{ Ractor.new{ task } }.each(&:take) }
}
```

```
master (095972e799):
       user     system      total        real
seq  1.618664   0.001916   1.620580 (  1.620620)
par 18.168252  16.840400  35.008652 (  9.452195)

This patch:
       user     system      total        real
seq  1.587542   0.000000   1.587542 (  1.587548)
par  1.950062   0.000000   1.950062 (  0.497173)
```




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