Pre-allocate slice in generic Map function

[jwilsson]

Hey!
Thanks for this great library, really helped me out with some scraping!

I noticed that the generic Map function just added the other week always appends, growing the slice as needed instead of pre-allocating it upfront.

Benchmark results from my M1:

Without pre-allocation:
BenchmarkMap-10                              	  238780	      5140 ns/op

With pre-allocation:
BenchmarkMap-10                              	  277968	      4317 ns/op

If you think it's not worth it, no hard feelings. Just thought it would be a fun, easy, optimization to make.

[mna]

Hey Jonathan,

Thanks for this! That behaviour actually predates the new Map addition as this is also how I had implemented the Selection.Map method. That's definitely worth it, I'll get that merged soon.

Martin