[Uri] Rewrite UriHelper.UnescapeString to remove unsafe code and improve perf

[stephentoub]

PR dotnet/corefx#41772 rewrote EscapeString with safe code, and it not only removed unsafe code from a key networking-exposed area of the system, it also resulting in performance benefits and cleaner code. We should look at doing the same thing for the other direction with UnescapeString. It's likely to be a bit more challenging, as it currently has more modes for configuration.

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