Replace usage of unshift with concat

[calumptrck]

I was able to get a better perf results by using Array.prototype.concat to create a new array with the additional args needed, instead of mutating the value being flattened twice with unshift and splice.

Results:

I also gave array.splice.call(array, i,1,...value) a shot which would transpile to array.call.apply(array.splice, [array, i, 1].concat(value)) but the benchmark was about the same.

edit: Tests are passing aside from verify optimizability -> with empty array, which wasn't passing originally on master.

[coveralls]

Coverage remained the same at 100.0% when pulling ba13cd5 on calumptrck:with-concat into 772d0dd on elidoran:master.