subscriber
clean up
arg resolver
start move to subscriber
add override support to make_subscriber

make_subscriber can take an existing subscriber and the some overrides
and produce a new subscriber that reuses what has not been overridden.
switch subscribe from observer to subscriber
fix a runtime crash
add variadic versions of  make_...
remove subscription from observer
test fixes
add support for operator | and well as operator >>
start switch to schedulable
compiles on clang at least.
restructuring tail-recursion to make range and for-loop perf similar when no marble schedules to the current thread.
added some comments and variadic subscribe
add some comments an clean up test structure
fix test scheduler - tests pass again
reduce race in subject
fix race *and* improve perf in subject

that is rare!
initialize current_generation