add to_string method for StringView

[Yu-zh]

This method will override the default trait implementation for to_string, which adds quotation marks to the result string and is not what we would like.

[coveralls]

Pull Request Test Coverage Report for Build 5228

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 85.791%

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Files with Coverage Reduction	New Missed Lines	%
string/view.mbt	1	81.03%

Totals
Change from base Build 5225:	-0.01%
Covered Lines:	5440
Relevant Lines:	6341

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💛 - Coveralls

[bobzhang]

386966d
cc @Yu-zh override Show::to_string instead