This commit ...

1. optimizes implementation to speed it up by about 15%
2. rescopes labels to `variable.other.member` as variable scope is what C# uses
   and this is what labels are for. Create named access to tuple members as if
   they were normal struct members.

Note:
1. `meta.mapping.key` should be used in mappings only.
2. We shouldn't mix up sequence and mapping scopes, although a mapping is just
   a sequence of key-value pairs.