Update to @most/core v1+

[semmel]

I had to patch the ProxyStream implementation, since the old relied on the fields .source and .sinks of MulticastSource being protected. In @most/core they're now private fields.

Also I removed the circular stream tests from attach, since

• I did not get circular streams to work with stream failures,
• I think its not widely used
• I don't think it ever worked in the old implementation (just recursively depend on an origin stream with a stream failure and observe "Stack size exceeded" events)

I added some more tests for attach though.

I renamed some variables in ProxyStream.ts for my own sanity (sorry though).

I guess this PR is a breaking change because

• the dependency @most/core is a breaking change anyway,
• the ProxyStream implementation changed quite a bit and I am not confident I recreated all the old behaviour (e.g. circular streams)

So at least it remains to bump the version number.