Add str_enc_copy_direct

This commit adds str_enc_copy_direct, which is like str_enc_copy but
does not check the frozen status of str1 and does not check the validity
of the encoding of str2. This makes certain string operations ~5% faster.

```ruby
puts(Benchmark.measure do
  100_000_000.times do
    "a".downcase
  end
end)
```

Before this patch:

```
  7.587598   0.040858   7.628456 (  7.669022)
```

After this patch:

```
  7.133128   0.039809   7.172937 (  7.183124)
```

0 files changed, 0 insertions, 0 deletions