Add map preallocation benchmarks

[scop]

Results from system in front of me:

BenchmarkMap/Size10NoPreallocate-4  	 1954923	       603.7 ns/op	     484 B/op	       3 allocs/op
BenchmarkMap/Size10Preallocate-4    	 2951250	       393.8 ns/op	     340 B/op	       2 allocs/op
BenchmarkMap/Size200NoPreallocate-4 	   71530	     15337 ns/op	   11270 B/op	      25 allocs/op
BenchmarkMap/Size200Preallocate-4   	  126673	      8216 ns/op	    5719 B/op	       8 allocs/op