Support parameter pack for Env threads

[pdillinger]

I tend to use std::thread because it supports functional static polymorphism with parameter packs. In other words, I can create a thread from an arbitrary function by providing arguments to the function, with full type checking, rather than casting through a single void *.

Filed at request of @mrambacher in #8225

This support could probably be generically bolted on in Env by wrapping a target with parameters in a heap-allocated std::function<void()>*, cast to void *, and in an unwrapper, cast back, executed, and deleted.