Add a plist type

[Fuco1]

• [x] Regular plist (:foo a :bar b)
• [ ] Add support for reading the &keys syntax from cl-defun
• [ ] As a "key" argument to functions, for example (propertize "string" 'a b 'c d), a and c are keys and b is a value... so the type is something like (function (string &rest [symbol value]) string). We need to add a good syntax for repeating a set of values. Take inspiration in macro instrumentation.