Split parameter-driven return types into overloads

Several methods have a boolean-ish argument that determines the return
type. Express each with literal `true`/`false` overloads instead of a
single signature with a loose union return:

- `Kernel#BigDecimal`: `exception: false` returns `nil` on a conversion
  failure, so `(?exception: true) -> BigDecimal` / `(exception: false) -> BigDecimal?`.
- `Thread::Queue#pop`: `non_block: true` raises `ThreadError` when the
  queue is empty, so it never returns `nil` — `(true) -> E` /
  `(?false, ?timeout:) -> E?`.

Author: zonuexe

core/thread.rbs

@@ -1719,7 +1719,8 @@ class Thread::Queue[E = untyped] < Object
   # If `timeout` seconds have passed and no data is available `nil` is returned.
   # If `timeout` is `0` it returns immediately.
   #
-  def pop: (?boolish non_block, ?timeout: _ToF?) -> E?
+  def pop: (?false non_block, ?timeout: _ToF?) -> E?
+         | (true non_block) -> E
 
   # <!--
   #   rdoc-file=thread_sync.c

stdlib/bigdecimal/0/big_decimal.rbs

@@ -1238,7 +1238,8 @@ module Kernel
   # Raises an exception if `value` evaluates to a Float and `digits` is larger
   # than Float::DIG + 1.
   #
-  def self?.BigDecimal: (real | string | BigDecimal initial, ?int digits, ?exception: bool) -> BigDecimal
+  def self?.BigDecimal: (real | string | BigDecimal initial, ?int digits, ?exception: true) -> BigDecimal
+                      | (real | string | BigDecimal initial, ?int digits, exception: false) -> BigDecimal?
 end
 
 %a{annotate:rdoc:skip}