speed up string decoding

This speeds up string decoding by using `rb_str_substr` rather than
`rb_str_new`.  `rb_str_substr` will create shared strings, so it will
avoid string copying like `rb_str_new` will do.

Before this patch:

```
[aaron@TC bert (master)]$ ruby -I lib:test bench/decode_bench.rb
                    user     system      total        real
BERT C Extension Decoder
BERT tiny       0.000000   0.000000   0.000000 (  0.000574)
BERT small      0.010000   0.010000   0.020000 (  0.014938)
BERT large     13.990000  11.640000  25.630000 ( 25.892584)
BERT complex    0.030000   0.010000   0.040000 (  0.033596)
```

After this patch:

```
[aaron@TC bert (master)]$ ruby -I lib:test bench/decode_bench.rb
                    user     system      total        real
BERT C Extension Decoder
BERT tiny       0.000000   0.000000   0.000000 (  0.000563)
BERT small      0.010000   0.000000   0.010000 (  0.008701)
BERT large      6.180000   0.040000   6.220000 (  6.299307)
BERT complex    0.060000   0.000000   0.060000 (  0.070287)
```

Author: tenderlove

ext/bert/c/decode.c

@@ -34,7 +34,9 @@ static VALUE rb_cTuple;
 
 struct bert_buf {
 	const uint8_t *data;
+	const uint8_t *start;
 	const uint8_t *end;
+	VALUE rb_buf;
 };
 
 static VALUE bert_read_invalid(struct bert_buf *buf);
@@ -300,7 +302,7 @@ static VALUE bert_read_bin(struct bert_buf *buf)
 	length = bert_buf_read32(buf);
 
 	bert_buf_ensure(buf, length);
-	rb_bin = rb_str_new((char *)buf->data, length);
+	rb_bin = rb_str_substr(buf->rb_buf, buf->data - buf->start, length);
 	buf->data += length;
 
 	return rb_bin;
@@ -512,6 +514,8 @@ static VALUE rb_bert_decode(VALUE klass, VALUE rb_string)
 
 	Check_Type(rb_string, T_STRING);
 	buf.data = (uint8_t *)RSTRING_PTR(rb_string);
+	buf.start = buf.data;
+	buf.rb_buf = rb_string;
 	buf.end = buf.data + RSTRING_LEN(rb_string);
 
 	bert_buf_ensure(&buf, 1);