High-level child-process spawning with builder-style configuration.

Wraps std/sys/process with a fluent API for constructing argv, capturing stdout/stderr through pipes, and waiting for the child to exit.

Example

{ Command } :: import "std/process/command";
{ Exception, AnyError } :: import "std/error";

main :: (fn(io : Io, exn : Exception) -> unit)({
  cmd := Command.new(`echo`);
  cmd.arg(`hello`);
  cmd.arg(`world`);
  out := io.await(cmd.output(io), { io, exn });
  assert(out.status.success(), "echo should succeed");
});