String#gsub shouldn't allocate so many Strings in its loop

### Summary

`rb_reg_search()` [allocates (dups) a String to attach to the backreference object](https://github.com/ruby/ruby/blob/trunk/re.c#L1453). If `#gsub` has been passed 2 arguments (not Enumerator form) and the second argument is a String, then it shouldn't make these allocations when calling `rb_reg_search()` inside it's loop.

## Example

```ruby
# gsub-allocates-too-much.rb
require File.join(__dir__, "lib", "allocation_stats")

def puts_object_list(name, stats)
  objects = stats.allocations.group_by(:sourcefile, :sourceline, :class).all.
    values.flatten.map(&:object).
    map {|o| o.is_a?(String) ? "#{o.inspect}<#{o.encoding.to_s}>" : o.inspect }
  puts "#{name} #{objects.flatten.size} new objects:"
  objects.group_by(&:hash).values.each { |ary| puts "#{ary.join(", ")}" }
end

slash = '/'; underscore = '_'; colon = ':' # allocate before the trace
str = "12:34:45:67"
stats = AllocationStats.trace { str.gsub(colon, underscore) }
puts '> "12:34:45:67".gsub(":", "_")'
puts_object_list("gsub substitutes 3x times:", stats)
```

```
$ ruby ../allocation_stats/gsub-allocates-too-much.rb
> "12:34:45:67".gsub(":", "_")
gsub substitutes 3x times: 12 new objects:
"12:34:45:67"<UTF-8>, "12:34:45:67"<UTF-8>, "12:34:45:67"<UTF-8>, "12:34:45:67"<UTF-8>
"12_34_45_67"<UTF-8>
":"<ASCII-8BIT>, ":"<ASCII-8BIT>
":"<US-ASCII>, ":"<US-ASCII>
#<MatchData ":">
#<MatchData nil>
/:/
```

The Strings that are copies of the original String are all unnecessary (except one).

## The Fix

The fix involves allocating the `str` attribute of the backreference object only when necessary. In order to do this without changing the signature of `rb_reg_search()`, this patch changes `rb_reg_search()` to wrap a new function `rb_reg_search0()`. So no calls to `rb_reg_search()` need to change, and `str_gsub()` changes just two calls into `rb_reg_search0()` to avoid the allocations.

## Impact

The impact of this fix is primarily faster garbage collection. I have two "real world" examples:

* ActiveRecord sqlite3 specs: total time in GC reduced from 11.2s to 10.4s (7% savings).
* Mail gem specs: total time in GC reduced from 0.220s to 0.215s (2% savings).

These numbers bounced around a lot though. I'm open to better benchmarking suggestions. I used ActiveRecord and Mail for real world examples of `#gsub`, where realistic Strings are gsubbed.