Implemented copy constructors for SEXP types

[codecov[bot]]

Codecov Report

Base: 99.70% // Head: 99.70% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (ac005db) compared to base (ec6bcc6). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #20 +/- ## ======================================= Coverage 99.70% 99.70% ======================================= Files 9 9 Lines 668 682 +14 ======================================= + Hits 666 680 +14 Misses 2 2 ``` | [Impacted Files](https://codecov.io/gh/rnpgp/sexp/pull/20?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rnpgp) | Coverage Δ | | |---|---|---| | [include/sexp/sexp.h](https://codecov.io/gh/rnpgp/sexp/pull/20/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rnpgp#diff-aW5jbHVkZS9zZXhwL3NleHAuaA==) | `100.00% <100.00%> (ø)` | | | [src/sexp-object.cpp](https://codecov.io/gh/rnpgp/sexp/pull/20/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rnpgp#diff-c3JjL3NleHAtb2JqZWN0LmNwcA==) | `98.70% <100.00%> (+0.15%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rnpgp). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rnpgp)

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