Support renaming stream symbols

[jvalkeal]

If you write multiple streams, i.e. for taps, there are references so it'd be nice to support renaming these. For example in:

main = time | transform | log
tap1 = :main.time > filter | log
tap2 = :main.transform > scriptable-transform | log
tap3 = :tap2.scriptable-transform > filter | log

it should be possible to rename main.