This PR adds in support for the %a{warning: ...} annotation, which can be attached to constants, methods, or global variables.

Motivating Usage

This is the usage that motivated it (from ruby/rbs#2987), slightly simplified:

class Array[E]
  def min: () { (E a, E b) -> Comparable::_CompareToZero } -> E?
         | # ... other branches ...
end

This is the correct signature—the return value of the block cannot return nil. However, most <=> implementations do return nil, and having to do a kludge (eg array.min { |a,b| (a.foo <=> b.foo) || raise("never happens") }) is quite un-ergonomic, awkward, and can potentially break backwards-compat.

The solution is to add in a warning annotation:

class Array[E]
  def min: () { (E a, E b) -> Comparable::_CompareToZero } -> E?
         | %a{warning: returning nil from comparisons always raises} () { (E a, E b) -> Comparable::_CompareToZero? } -> E?
         | # ... other branches ...
end