Optimize ninjaString.ValueWithEscaper

ninjaString.ValueWithEscaper is a relatively hot function,
rewrite it with strings.Builder to avoid repeated string
concatenation, which requires an allocation each time.

Before:
BenchmarkNinjaString_Value/constant/1-72       	100000000	        11.9 ns/op
BenchmarkNinjaString_Value/constant/10-72      	100000000	        18.9 ns/op
BenchmarkNinjaString_Value/constant/100-72     	50000000	        22.1 ns/op
BenchmarkNinjaString_Value/constant/1000-72    	30000000	        39.3 ns/op
BenchmarkNinjaString_Value/variable/1-72       	20000000	        95.1 ns/op
BenchmarkNinjaString_Value/variable/10-72      	10000000	       223 ns/op
BenchmarkNinjaString_Value/variable/100-72     	 3000000	       437 ns/op
BenchmarkNinjaString_Value/variable/1000-72    	 2000000	       948 ns/op
BenchmarkNinjaString_Value/variables/1-72      	10000000	       161 ns/op
BenchmarkNinjaString_Value/variables/2-72      	 5000000	       368 ns/op
BenchmarkNinjaString_Value/variables/3-72      	 3000000	       560 ns/op
BenchmarkNinjaString_Value/variables/4-72      	 2000000	       795 ns/op
BenchmarkNinjaString_Value/variables/5-72      	 1000000	      1004 ns/op
BenchmarkNinjaString_Value/variables/10-72     	 1000000	      2275 ns/op
BenchmarkNinjaString_Value/variables/100-72    	   50000	     39667 ns/op
BenchmarkNinjaString_Value/variables/1000-72   	    1000	   2146592 ns/op

After:
BenchmarkNinjaString_Value/constant/1-72       	200000000	        11.3 ns/op
BenchmarkNinjaString_Value/constant/10-72      	100000000	        17.2 ns/op
BenchmarkNinjaString_Value/constant/100-72     	50000000	        21.7 ns/op
BenchmarkNinjaString_Value/constant/1000-72    	30000000	        38.3 ns/op
BenchmarkNinjaString_Value/variable/1-72       	20000000	        91.8 ns/op
BenchmarkNinjaString_Value/variable/10-72      	10000000	       199 ns/op
BenchmarkNinjaString_Value/variable/100-72     	 5000000	       377 ns/op
BenchmarkNinjaString_Value/variable/1000-72    	 2000000	       855 ns/op
BenchmarkNinjaString_Value/variables/1-72      	10000000	       141 ns/op
BenchmarkNinjaString_Value/variables/2-72      	 5000000	       312 ns/op
BenchmarkNinjaString_Value/variables/3-72      	 5000000	       362 ns/op
BenchmarkNinjaString_Value/variables/4-72      	 3000000	       417 ns/op
BenchmarkNinjaString_Value/variables/5-72      	 2000000	       621 ns/op
BenchmarkNinjaString_Value/variables/10-72     	 2000000	       837 ns/op
BenchmarkNinjaString_Value/variables/100-72    	  200000	      9141 ns/op
BenchmarkNinjaString_Value/variables/1000-72   	   20000	     95094 ns/op

Test: ninja_strings_test.go
Change-Id: I6c61e747d8e67f7f1e6cff0cc0c705745301a35f