Add examples and benchmarks

[srid]

For each example, benchmark the script size and cpu/mem cost as returned by Plutus evaluator.

Should eventually benchmark the individual DSL functions as well.

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• Add examples/ and explain them in the README
• Have cabal run or something spit out benchmarks flake: add checks.benchmark and apps.benchmark #16
• Include benchmarks in CI
• Triage individual function benchmark, and CI comparison (example)

[srid]

Closing this in favour of the atomic issues #27 and #28