Add new ArrayConcatLiteral cop to replace a.concat([1, 2, 3]) with a.push(1, 2, 3)

[amomchilov]

Fixes #434

This PR is stacked on top of commits in #433.

ArrayConcatLiteral synergizes with ArrayPushSingle, so array.concat([x]) will be simplified down to just array << x

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• Added tests.
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[amomchilov]

#434 (comment)