cli: solana-tokens, validate inputs gracefully (solana-labs#33926)

* cli: solana-tokens, refactor imports

* cli: solana-tokens, validate inputs gracefully

* change Allocation struct field types to simplify things

* fix typos, apply some review suggestions

* preserve backward compatibility for public APIs

* apply latest review suggestions

* address next batch of review comments

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Co-authored-by: norwnd <norwnd>