Memoize URL parsing while calculating absolute soruces

[jasonLaster]

URL parsing is the slowest part of instantiating a source map consumer. Many of the URLs parsed are the same, which means that memoizing can have a nice performance improvement.

In this example, memoizing URL parsing saves ~200ms (475ms --> 287ms) . old --> new

[coveralls]

Pull Request Test Coverage Report for Build 551

• 25 of 27 (92.59%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.007%) to 88.378%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/util.js	23	25	92.0%

Totals
Change from base Build 550:	-0.007%
Covered Lines:	841
Relevant Lines:	930

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💛 - Coveralls