Speed up setting the backref match object

This patch speeds up setting the backref match object by avoiding some
memcopies.  Take the following code for example:

```ruby
"hello world" =~ /hello/
p $~
```

When the RE matches the string, we have to set the Match object in the
backref global.  So we would allocate a match object[^1] and use
`rb_reg_region_copy`[^2] to make a deep copy of the stack allocated
`re_registers` struct[^3] in to the newly created Ruby object.  This
could possibly trigger GC[^4], and would allocate new memory.

This patch makes a shallow copy of the `re_registers` struct on to the
Match object allowing the match object to manage the `re_registers`
pointer and also avoiding some calls to `xmalloc` and some manual
memcopy.

Benchmark looks like this:

```ruby

require "benchmark/ips"

def test_re thing
  thing =~ /hello/
end

Benchmark.ips do |x|
  x.report("re hit") do
    test_re "hello world"
  end

  x.report("re miss") do
    test_re "world"
  end
end
```

Before this patch:

```
$ ruby -v test.rb
ruby 3.2.0dev (2022-07-27T22:29:00Z master 4ad69899b7) [arm64-darwin21]
Ignoring bcrypt-3.1.16 because its extensions are not built. Try: gem pristine bcrypt --version 3.1.16
Warming up --------------------------------------
              re hit   345.401k i/100ms
             re miss   673.584k i/100ms
Calculating -------------------------------------
              re hit      3.452M (± 0.5%) i/s -     17.270M in   5.002535s
             re miss      6.736M (± 0.4%) i/s -     34.353M in   5.099593s
```

After this patch:

```
$ ./ruby -v test.rb
ruby 3.2.0dev (2022-08-01T21:24:12Z less-memcpy 0ff2a56606) [arm64-darwin21]
Warming up --------------------------------------
              re hit   419.578k i/100ms
             re miss   673.251k i/100ms
Calculating -------------------------------------
              re hit      4.201M (± 0.7%) i/s -     21.398M in   5.093593s
             re miss      6.716M (± 0.4%) i/s -     33.663M in   5.012756s
```

Matches get faster and misses maintain the same speed

[^1]: https://github.com/ruby/ruby/blob/24204d54ab730791bfbd0cd66b8e12f0bd62ca5d/re.c#L1737
[^2]: https://github.com/ruby/ruby/blob/24204d54ab730791bfbd0cd66b8e12f0bd62ca5d/re.c#L1738
[^3]: https://github.com/ruby/ruby/blob/24204d54ab730791bfbd0cd66b8e12f0bd62ca5d/re.c#L1686
[^4]: https://github.com/ruby/ruby/blob/24204d54ab730791bfbd0cd66b8e12f0bd62ca5d/re.c#L981

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