Add a percent decoding fast-path

[karwa]

If the source data does not even contain a "%", we can trivially determine that percent-decoding is not required. Similarly, if it does not contain a "%" or "+", we can determine that form-decoding is not required.

This cuts the time to iterate path components (which often don't contain percent-encoding) by about 30-40%, and sometimes much more -- if the source is a String, we don't even need to copy. And it has essentially no impact on data which actually is encoded, because constructing a string from decoded contents is way more expensive than this check.

Before:

name                                    time        std        iterations
-------------------------------------------------------------------------
PathComponents.Iteration.Small.Forwards 1284.000 ns ±  49.23 %    1000000       <-----
PathComponents.Iteration.Small.Reverse  1255.000 ns ±  44.46 %    1000000       <-----
PathComponents.Iteration.Long.Forwards  5708.000 ns ±  29.53 %     223292       <-----
PathComponents.Iteration.Long.Reverse   5663.000 ns ±  28.37 %     224446       <-----
PathComponents.Append.Single            1604.000 ns ±  44.27 %     776674
PathComponents.Append.Multiple          2206.000 ns ±  46.02 %     557985
PathComponents.RemoveLast.Single         836.000 ns ±  69.40 %    1000000
PathComponents.RemoveLast.Multiple       863.000 ns ±  46.90 %    1000000
PathComponents.ReplaceSubrange.Shrink   1616.000 ns ±  46.60 %     774023
PathComponents.ReplaceSubrange.Grow     2695.000 ns ±  38.74 %     465685

name                                    time        std        iterations
-------------------------------------------------------------------------
URLEncoded.String.urlEncoded            9156.000 ns ±  28.04 %     142419
URLEncoded.String.urlDecoded            4634.000 ns ±  30.95 %     275012
URLEncoded.String.urlDecoded.notEncoded 2114.000 ns ±  48.68 %     583163       <-----

After:

name                                    time        std        iterations
-------------------------------------------------------------------------
PathComponents.Iteration.Small.Forwards  878.000 ns ±  36.19 %    1000000       <-----
PathComponents.Iteration.Small.Reverse   861.000 ns ±  34.98 %    1000000       <-----
PathComponents.Iteration.Long.Forwards  3908.000 ns ±  17.90 %     342676       <-----
PathComponents.Iteration.Long.Reverse   3768.000 ns ±  18.62 %     354951       <-----
PathComponents.Append.Single            1562.000 ns ±  27.47 %     804555
PathComponents.Append.Multiple          2140.000 ns ±  23.04 %     631751
PathComponents.RemoveLast.Single         698.000 ns ±  33.81 %    1000000
PathComponents.RemoveLast.Multiple       714.000 ns ±  32.44 %    1000000
PathComponents.ReplaceSubrange.Shrink   1642.000 ns ±  24.41 %     781916
PathComponents.ReplaceSubrange.Grow     2658.000 ns ±  21.13 %     500778

name                                    time        std        iterations
-------------------------------------------------------------------------
URLEncoded.String.urlEncoded            8775.000 ns ±  26.12 %     138089
URLEncoded.String.urlDecoded            4417.000 ns ±  36.92 %     290181
URLEncoded.String.urlDecoded.notEncoded  403.000 ns ±  68.14 %    1000000       <-----