New interface function "compose"
Using compose for filter
Unlazy tests, other types for rx
Simple composing of Rx works
Map is not lazy, "remain" typoo is corrected
Map does not subscribe directly (sync call)
Filter and map are synchronous and short thanks to macros
Calling composed functions with one or two arguments
First implementation of take with new_accu
Added new_accu for ignore and on_error
Added new_accu
Synchronous reducer functions
Use return value construction by macros
Handle composition with accus correctly
Fold the past uses the correct double accumulators
Emit a value by on_completed
Take does not emit a value at complete
Define sum via Sync.sum
Variations on halt and completed
How to define all three cases properly?