followup performance optimization to 1308

[cdisselkoen]

Description of changes

Inspired by this thread in #1308. Avoids repeatedly calling ast::Name::parse_unqualified_name() during EST parsing. On my machine (a different machine than the one I reported numbers on in #1308), the performance of our deeply_nested_est benchmark goes from 71.6 µs before this PR, to 63.3 µs after this PR, an 11% improvement.

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The change in this PR is (choose one, and delete the other options):

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• Does not update the CHANGELOG because my change does not significantly impact released code.

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• Does not require updates because my change does not impact the Cedar language specification.

[cdisselkoen]

Updated performance numbers after my latest push: now a 14% improvement on the deeply_nested_est benchmark from before this PR to after this PR