Improve performance of string interpolation

This patch will add pre-allocation in string interpolation.
By this, unecessary capacity resizing is avoided.

For small strings, optimized `rb_str_resurrect` operation is
faster, so pre-allocation is done only when concatenated strings
are large.  `MIN_PRE_ALLOC_SIZE` was decided by experimenting with
local machine (x86_64-apple-darwin 16.5.0, Apple LLVM version
8.1.0 (clang - 802.0.42)).

String interpolation will be faster around 72% when large string is created.

* Before
  ```
  Calculating -------------------------------------
  Large string interpolation
                            1.276M (± 5.9%) i/s -      6.358M in   5.002022s
  Small string interpolation
                            5.156M (± 5.5%) i/s -     25.728M in   5.005731s
  ```

* After
  ```
  Calculating -------------------------------------
  Large string interpolation
                            2.201M (± 5.8%) i/s -     11.063M in   5.043724s
  Small string interpolation
                            5.192M (± 5.7%) i/s -     25.971M in   5.020516s
  ```

* Test code
  ```ruby
  require 'benchmark/ips'

  Benchmark.ips do |x|
    x.report "Large string interpolation" do |t|
      a = "Hellooooooooooooooooooooooooooooooooooooooooooooooooooo"
      b = "Wooooooooooooooooooooooooooooooooooooooooooooooooooorld"

      t.times do
        "#{a}, #{b}!"
      end
    end

    x.report "Small string interpolation" do |t|
      a = "Hello"
      b = "World"

      t.times do
        "#{a}, #{b}!"
      end
    end
  end
  ```

[Fix GH-1626]
From: Nao Minami <south37777@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60320 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_string.rb

@@ -620,6 +620,11 @@ CODE
     assert_raise(RuntimeError) { 'foo'.freeze.concat('bar') }
   end
 
+  def test_concat_literals
+    s="." * 50
+    assert_equal(Encoding::UTF_8, "#{s}x".encoding)
+  end
+
   def test_count
     a = S("hello world")
     assert_equal(5, a.count(S("lo")))