Decode into slice reuses element data between len and cap

#306 added logic to retain blocks when decoding, which allows nice handling of default values. However, when creating #306, I wasn't aware of golang/go#21092. TL;DR when decoding slices, all elements should be reset instead of merged, to avoid confusion about created data structures. Currently HCL decoder follows the same behavior as `encoding/json`. Perhaps this could be fixed in HCL faster than in JSON decoder, but I'm not sure about this.

# References
Are there any other GitHub issues (open or closed) that should
be linked here? For example:
- #306

Start by reviewing the slice-decoding changes from #306 and comparing the current behavior with Go's encoding/json decoder. Confirm that decoding resets every slice element instead of merging retained data; the issue provides no specific files or tests to run.

Written by the indexing model from the issue text.