Added capacity to slice creation, when capacity is known

[vivekv96]

Simple optimisation to reduce memory allocations and copying(while appending).

[josephlr]

Nice fix, did you find this by reading the code? Or though some tooling?

[vivekv96]

@josephlr, I found this by reading the code, but I am thinking of writing a small script over the weekend that can help with these kind of micro optimisations.
Something that probably does a grep on open source Go projects and finds slices that are being used immediately without a capacity, not sure how the script should work exactly but should be a fun side project.