rb_str_buf_append: add a fast path for ENC_CODERANGE_VALID

If the RHS has valid encoding, and both strings have the same
encoding, we can use the fast path.

However we need to update the LHS coderange.

```
compare-ruby: ruby 3.2.0dev (2022-07-21T14:46:32Z master cdbb9b8555) [arm64-darwin21]
built-ruby: ruby 3.2.0dev (2022-07-25T07:25:41Z string-concat-vali.. 11a2772bdd) [arm64-darwin21]
warming up...

|                    |compare-ruby|built-ruby|
|:-------------------|-----------:|---------:|
|binary_concat_7bit  |    554.816k|  556.460k|
|                    |           -|     1.00x|
|utf8_concat_7bit    |    556.367k|  555.101k|
|                    |       1.00x|         -|
|utf8_concat_UTF8    |    412.555k|  556.824k|
|                    |           -|     1.35x|
```

1 files changed, 13 insertions, 13 deletions

diff --git a/benchmark/string_concat.yml b/benchmark/string_concat.yml
index 0ff1dc25b6..b8a69ed909 100644
--- a/benchmark/string_concat.yml
+++ b/benchmark/string_concat.yml
@@ -1,9 +1,9 @@
 prelude: |
   CHUNK = "a" * 64
-  BCHUNK = "a".b * 64
+  UCHUNK = "é" * 32
   GC.disable # GC causes a lot of variance
 benchmark:
-  binary_concat_utf8: |
+  binary_concat_7bit: |
     buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
@@ -13,17 +13,7 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
-  binary_concat_binary: |
-    buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-  utf8_concat_utf8: |
+  utf8_concat_7bit: |
     buffer = String.new(capacity: 4096, encoding: Encoding::UTF_8)
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
@@ -33,3 +23,13 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+  utf8_concat_UTF8: |
+    buffer = String.new(capacity: 4096, encoding: Encoding::UTF_8)
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK