Rename Observable.Amb to Race

[neuecc]

Amb is a name that has been continuously used since the original Rx, with its etymology coming from LISP's 'amb', but it's not an intuitive name for its behavior.
I think it would be more appropriate to change it to something like Race.
It was my mistake not to change the name during R3's initial release.
For the transition, I suggest adding Race while marking Amb as [Obsolete].
What do you think?

[neuecc]

v.1.3.0 changed.