Cyclic extends generics

[kaleidawave]

A generic can reference itself in the extends clause.

type X<T> = T extends (infer U extends { a: U })

This seems to be true for all generics: infer T, mapped keys, on functions and structures

The following needs implementing:

• in subtyping: specialisation needs to happen first then extends subtype checking
• during synthesis:
  • first pass: needs to register these
  • second pass: needs to check cycles (via disjoint in Improvements to properties + intrinsic types #157) + check constraints + reset constraints