rewrite mock_backtrace to use define_singleton_method.
add Pry::LastException.

pry adds behavior(methods) to the exception it returns at _pry_.last_exception
in a repl but when the exception has been passed along as a frozen object ruby
will raise an exception and pry fails to function as usual.

the commit changes that behavior so that the exception is wrapped in an instance of
Pry::LastException who is a subclass of BasicObject. Pry::LastException is a copy&paste
of what was defined dynamically in pry_instance.rb but it allows for frozen exceptions
to remain frozen and still provides the methods pry dynamically defined on an exception
object before. through a wrapper class pry also no longer modifies an exception object
that it doesn't own.
update CHANGELOG.md
Merge pull request #1145 from robgleeson/fix-last_exception

Add Pry::LastException class wrapping last_exception