Re-implement ticker prefixes using key: Seq[String] concatenation

[lihaoyi]

This re-implements the ticker log prefixes that were left out in #3577, both for meta-build prefixes and nested-evaluation prefixes, e.g. the log prefix now looks like [build.mill-1] [mill-build/build.mill-1] etc.

• We no longer need dynamicTickerPrefix, because we now perform this context management by wrapping nested PrefixLoggers

• Required some gross mangling to consolidate Logger and ColorLogger interfaces in order to make things properly stackable

• We needed to move the aggregation of keys to a top-down process, rather than bottom up, since it is not easy to take an already prefixed line [1] my logs line and append to the prefix [1-2] my logs line. Thus we need to make sure the log line is properly prefixed at the bottom-most logger, and the log line is passed up via unprefixedSystemStreams to avoid redundant prefixing