TP's constructors takes start date instead of end date - this is faster and more intuitive, especially when using DT or DTO
* Fix collections to make them compile

* Implement IReadOnlyCollection<T> interface for read-only maps/series
* Fix FSharpx dependencies. Just copy relevant code, avoid dependencies when possible

TODO
* Add all old collections tests and make a benchmark that runs all data structures (for not to comment/uncomment anymore)
* SortedMap optimization (first/last), try

BACKLOG
* Test TimePeriod with FsCheck-like millions of inputs