Rearchitect and simplify

# Rearchitect and Simplify

This collapses the project code down to two files, `counter.go` and `counter_test.go`, and swaps the underlying list container for a `map`.

Tests look good:

```
$ go test
PASS
ok      counter 0.014s
$ go test --race
PASS
ok      counter 1.301s
```