Samukweku/pivot longer simple

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• all forms of pivot_longer handled by two major functions reshape_by_spec and reshape_by_spec_others
• at its core, any pivot_longer function has to create a spec and pass to one of the two functions above.
• add lots of comments on the implementation logic (for future me or others)
• deprecate dropna - no perf difference if user does it, and is not needed within pivot_longer, plus we avoid the complexity of determing which NA rows to drop

This PR improves pivot_longer.

Please tag maintainers to review.

• @ericmjl

[ericmjl]

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