Implement display_list

[thomastanck]

Resolves #379, and also changes the standard stringify of lists slightly, see eac6949.

The implementation does two passes over the list structure. It first performs a deep copy of the entire list structure, and at the same time identifies nodes which are list-like by constructing a ListObject for it. On the first pass, we keep all references as references to the original list structure for simplicity. In a second pass, it then replaces references to the old list structure, using set_head/tail to point these to the new list structure. We don't process the same pair more than once, so runtime complexity is linear in the number of pairs in the list structure.

The implementation assumes that the whole thing is a pure list structure, and there are no objects. To fix this for objects is not too tricky: we just have to add more special cases to the top of the visit function, to have the visit function recursively call object members, and also recreate objects during the second pass.

This PR also exposes a new interface similar to toReplString, that we use to implement the pretty printing. Now, if an object contains the properties replPrefix, replSuffix, and replArrayContents, it will assume the object is "array-like", and will stringify the contents and join them very similarly to how it handles pairs/arrays. This means formatting logic (e.g. multiline handling) is kept within stringify, and can be changed more easily if we need to.

This raises the possibility of perhaps pretty printing runes created by beside, etc. as those are also "array-like".

[coveralls]

Pull Request Test Coverage Report for Build 3805

• 69 of 69 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 83.364%

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Totals
Change from base Build 3784:	0.1%
Covered Lines:	6829
Relevant Lines:	7900

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💛 - Coveralls

[martin-henz]

Played a bit. Looks really solid. Good job @thomastanck ! Will try to find a reviewer.

[openorclose]

other than the infinite lists, let's have some tests of list of pairs, to make sure they display properly like

list(pair(1, 2), pair(3, 4));

[openorclose]

since it needs to be a function

Suggested change

	typeof v.replArrayContents !== 'undefined'
	typeof v.replArrayContents === 'function'

maybe can check that the prefix and suffix are strings instead too