Problem 6.
Slow prime number channel and 'Nth' helper.
Problem 6.
Much faster prime number tester.
Problem 7.
Problem 8.
Problem 9.
Problem 10.

Prime number generator still appears to be a bottleneck.
Updated prime number generator with sieve.

Similar benchmarks for 2M primes but better potential:
O(n log n log log n) vs O(n^2/(log n)^2).
Add helper for Max(chan int) and ChainMultiply(chan int, len).
Alpha-order.
New helpers: ChannelLength, Range, Merge.

Also: Tests.
Use array of *Composite in primes_sieve.
Problem 11.
Fix typo.