Improved performance of BSON::_decode_cstring

Decoding large BSON objects are a bit slower in Mango. I tried to speed up the function _decode_cstring, getting rid of excess memory copy using unpack.

Logs of NYTProf with the old version _decode_cstring:
```text
Profile of analyze.pl for 37.9s (of 42.1s), executing 1824734 statements and 861662 subroutine calls in 88 source files and 81 string evals.

50197	1	1	14.2s	29.5s	          Mango::BSON::_decode_cstring
100397	4	1	13.1s	13.1s	          Mango::BSON::CORE:unpack (opcode)
100368	2	1	2.37s	4.23s	           Mojo::Util::decode
18	3	1	1.78s	37.4s	          Mango::BSON::_decode_doc (recurses: max depth 3, inclusive time 111s)
50197	1	1	1.40s	37.4s	          Mango::BSON::_decode_value (recurses: max depth 3, inclusive time 78.7s)
100368	1	1	1.06s	1.48s	         Encode::utf8::decode_xs (xsub)
50171	1	1	1.04s	3.42s	          Mango::BSON::_decode_string
50203	3	2	758ms	758ms	Mango::BSON::Document::STORE
50198	7	2	424ms	553ms	          Mango::BSON::decode_int32
100368	1	1	417ms	417ms	     Encode::Encoding::renewed
100391	2	1	389ms	389ms	           Mojo::Util::_encoding
50180	4	3	297ms	297ms	Mango::BSON::Document::FETCH
17	1	1	292ms	292ms	             IO::Poll::_poll (xsub)
50188	3	2	210ms	210ms	Mango::BSON::Document::NEXTKEY
1	1	1	30.0ms	63.2ms	Mango::BSON::ObjectID::BEGIN@5
```
Logs of NYTProf with the new version _decode_cstring:
```text
Profile of analyze.pl for 18.7s (of 22.6s), executing 1824734 statements and 861662 subroutine calls in 88 source files and 81 string evals.


50197	1	1	7.93s	7.93s	          Mango::BSON::CORE:subst (opcode)
100368	2	1	2.13s	3.79s	           Mojo::Util::decode
18	3	1	1.54s	18.1s	          Mango::BSON::_decode_doc (recurses: max depth 3, inclusive time 52.9s)
50197	1	1	1.33s	18.1s	          Mango::BSON::_decode_value (recurses: max depth 3, inclusive time 40.0s)
50171	1	1	997ms	3.34s	          Mango::BSON::_decode_string
100368	1	1	954ms	1.32s	         Encode::utf8::decode_xs (xsub)
50197	1	1	849ms	10.7s	          Mango::BSON::_decode_cstring
50203	3	2	663ms	663ms	Mango::BSON::Document::STORE
50198	7	2	394ms	492ms	          Mango::BSON::decode_int32
100368	1	1	369ms	369ms	     Encode::Encoding::renewed
100391	2	1	336ms	337ms	           Mojo::Util::_encoding
17	1	1	302ms	302ms	             IO::Poll::_poll (xsub)
50180	4	3	299ms	299ms	Mango::BSON::Document::FETCH
50188	3	2	210ms	210ms	Mango::BSON::Document::NEXTKEY
50200	3	1	98.1ms	98.1ms	          Mango::BSON::CORE:unpack (opcode)
```