[Placeholder] built-in HSL IO

[fredemmott]

A formal HIP needs to be written for this, to get consensus on design decisions, especially surrounding generators.

This needs to be built-in to unblock work on removal of globals and resources, in particular, STDIN, STDOUT, and so on. It also has side-effects on the behavior of var_dump(), print() etc due to setting the underlying FD to be non-blocking.