Abort if subtyping fails to (possible) infinite loop

[soutaro]

Type checking "" =~ "" doesn't stop by infinitely recursive subtyping check.

# Simplified RBS excerpt

interface _MatchAgainst[T]
  def =~: (String) -> T
end

class String
  def =~: (Regexp) -> Integer?
        | [T] (_MatchAgainst[T]) -> T
end

# Ruby code

"" =~ ""

# Subtyping trace

String <: _MatchAgainst[T_1]                           # T_1 is a fresh type variable
  =~
    [T] (_MatchAgainst[T]) -> T <: (String) -> T_1     # Expand =~ method types
      (_MatchAgainst[T_2]) -> T_2 <: (String) -> T_1   # Expand generics with fresh type variable T_2
        String <: _MatchAgainst[T_2]                   # The first subtyping relationship but with T_2, and continues
        T_2 <: T_1

Instead of causing SystemStackError, let subtyping check fail with new LoopAbort error.

[soutaro]

The default limit is 100, but you can override with $STEEP_SUBTYPING_ABORT_LIMIT environment variable.
Note that this is only for troubleshooting and not expecting to be configured by the user for daily uses.

[soutaro]

Changed the default limit to 50 because 100 was too big for Windows.