chore: Modernize lib/source-maps.js

[coreyfarrell]

• Convert to class
• Prefer const over var
• Use Object.entries()

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This eliminates the last use of prototype based class declaration in nyc, git grep prototype only shows the one place where a test replaces one of the nyc methods.

[coveralls]

Coverage decreased (-0.02%) to 98.482% when pulling 5be3e04 on coreyfarrell:modernize-source-maps into f645762 on istanbuljs:master.

[coreyfarrell]

Not an actual drop in coverage, this removes coverable lines/statements from lib/source-maps.js, for example SourceMaps.prototype.cachedPath = function ... was previously a coverage hit.