Pass through the implementation of more index map operations.

[loganfsmyth]

Indexed maps have a lot of duplication with basic maps right now, and I can't see a good reason for it, so this is an effort to normalize all of that logic on top of the basic map implementation.

This PR also includes support for computeColumnSpans on indexed maps, so that the two types of consumers have a consistent API again.

Beyond the expected stuff, this PR also includes one breaking change. The basic consumer's eachMapping function now returns lastGeneratedColumn as Infinity instead of null when you've called computeColumnSpans and there was no span end. Before this, it was null, but setting it to Infinity keeps the API consistent with generatedPositionFor and allGeneratedPositionsFor's lastColumn value.

[tromey]

I don't really know much about this area so I think I will leave it to Nick.

[coveralls]

Pull Request Test Coverage Report for Build 516

• 53 of 55(96.36%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+4.2%) to 88.063%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/source-map-consumer.js	53	55	96.36%

Totals
Change from base Build 505:	4.2%
Covered Lines:	876
Relevant Lines:	972

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💛 - Coveralls

[fitzgen]

Looks great! Thanks @loganfsmyth :)