Add a "list" type of "repeating elements"

[Fuco1]

For example, something like a list which is always a sequence of symbol and an int could be (repeat symbol int). This would typecheck (a 1 b 2) but not (1 2 3) or (a 1 2 b).

We need to think about the syntax and the common cases.

• A plist as a bag of whatever but with keyword keys: (repeat keyword mixed)
• Arguments to something like propertize.
• List of even length: (repeat mixed mixed)

If at all possible we should try to reuse some syntax from either customize or macro instrumentation.