Use a mock sentinel for MARK instead of a gensym

[gilch]

Gensyms just resolve to strings, and shouldn't be used literally anywhere they might collide with adversarial input, which commonly come in the form of unsanitized strings. User input could theoretically change where a MARK is, altering behavior.

sentinels normally shouldn't be used outside of tests, but the standalone property limits my options. At run time a simple object() would do, but unpickling the same object() created at read time twice wouldn't preserve equality in this case. A sentinel can do that.

I've used getattr to create a sentinel with a non-identifier string, which are almost never used, so this is unlikely to interfere with tests. For good measure, I prepended hissp. to the ] as an ad-hoc namespace.

[codecov[bot]]

Codecov Report

Merging #216 (3e059e2) into master (4874223) will not change coverage. The diff coverage is n/a.

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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          710       710           
  Branches       109       109           
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  Hits           710       710           

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