StringFormatter

General purpose string formatter, with the following features:

1. On all Showables, show is called instead of toString
2. Exceptions raised by a show are handled by falling back to toString.
3. Sequences can be formatted using the desired separator between two % signs, eg i"myList = (${myList}%, %)"
4. Safe handling of multi-line margins. Left margins are skipped om the parts of the string context before inserting the arguments. That way, we guard against accidentally treating an interpolated value as a margin.