Refactor to support Crystal 1.1.1

[noahlh]

First off, @akitaonrails, thank you for creating this! It's exactly what I needed for my project.

I upgraded to Crystal 1.1.1 and everything broke, though :( I saw also that #3 refers to the same issue.

So I dug in, and in I did a small refactor for compatibility with Crystal 1.1.1

The issue is that the Crystal team made Channel::StrictReceiveAction private in #9564 and, in general in other discussions, has discouraged folks from using internal Channel implementation details. So really, we should just be using Channel#send and Channel#receive.

So I modified the code here slightly to do that. Instead of an array of Channels (one for each worker), I'm using a single communication channel and the array stores references to the workers themselves.

This is a first stab at a solution and I'm sure it can be improved / cleaned-up, but it works! I'm happily chugging along again.