Adds ion-hash-test submodule and applies fixes to BigListOfNaughtyStringTests.ts

[pbcornell]

• Updates BigListOfNaughtyStringsTests to properly recognize the 'invalid_ion::' prefix, and to use 'md5' for hashing
  • The runTest() method previously constructed a TestValue object based on testString, from which any ion:: or invalid_ion:: prefix has been removed, so the value of TestValue.isValidIon is incorrect. These changes address that bug
  • Also changes the hash algorithm from 'identity' to 'md5' to enable comparison with other Ion Hash implementations
• Adds the ion-hash-test submodule and updates README.md
• Updates BigListOfNaughtyStringsTests.ts to use ion-hash-test/big_list_of_naughty_strings.txt
• Removes tests/big-list-of-naughty-strings.txt

I've also opened #23 to update IonHashTests.ts to use ion-hash-test/ion_hash_tests.ion once $0 is supported.

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