chore(performance): Improve performance for adding RemoveExtraSlash flag

[appleboy]

See the detailed benchmark: #2153

before:

#GithubAPI Routes: 203
   Gin: 58512 Bytes

#GPlusAPI Routes: 13
   Gin: 4384 Bytes

#ParseAPI Routes: 26
   Gin: 7776 Bytes

#Static Routes: 157
   Gin: 34936 Bytes

goos: darwin
goarch: amd64
pkg: github.com/julienschmidt/go-http-routing-benchmark
BenchmarkGin_Param        	11682692	       107 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param5       	 5853684	       205 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param20      	 2497855	       484 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParamWrite   	 6599636	       180 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubStatic 	 9790374	       125 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubParam  	 4965781	       244 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubAll    	   24424	     49748 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusStatic  	14304789	        86.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusParam   	 6814308	       174 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlus2Params 	 4847437	       253 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusAll     	  504351	      2302 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseStatic  	12430803	        99.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseParam   	10323856	       119 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Parse2Params 	 7512644	       159 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseAll     	  305469	      3961 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_StaticAll    	   37492	     30986 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/julienschmidt/go-http-routing-benchmark	23.774s

after:

#GithubAPI Routes: 203
   Gin: 58512 Bytes

#GPlusAPI Routes: 13
   Gin: 4384 Bytes

#ParseAPI Routes: 26
   Gin: 7776 Bytes

#Static Routes: 157
   Gin: 34936 Bytes

goos: darwin
goarch: amd64
pkg: github.com/julienschmidt/go-http-routing-benchmark
BenchmarkGin_Param        	15443652	        78.9 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param5       	 9414763	       127 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param20      	 4002259	       298 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParamWrite   	 7826979	       151 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubStatic 	11584101	       103 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubParam  	 7606005	       158 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubAll    	   36332	     34051 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusStatic  	12782667	        79.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusParam   	12041554	       103 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlus2Params 	 9379024	       128 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusAll     	  813216	      1394 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseStatic  	15016789	        77.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseParam   	15004710	        82.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Parse2Params 	11183847	       109 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseAll     	  448926	      2553 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_StaticAll    	   51507	     23745 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/julienschmidt/go-http-routing-benchmark	22.477s

ref issue: #1817 and #1644

cc @thinkerou @dmarkham @QianChenglong

[codecov]

Codecov Report

Merging #2159 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2159      +/-   ##
==========================================
+ Coverage   98.61%   98.61%   +<.01%     
==========================================
  Files          40       40              
  Lines        2238     2240       +2     
==========================================
+ Hits         2207     2209       +2     
  Misses         16       16              
  Partials       15       15

Impacted Files	Coverage Δ
gin.go	98.29% <100%> (+0.01%)	⬆️

---

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[thinkerou]

LGTM