Enhancements: Error given non-spied object, More efficient and safer reinitialization of spies

[igrep]

I was planning to include rspec-crispy in this pull request. I actually implemented rspec-crispy in this pull request, but I found it's less flexible than I expected. Then I decided not to include rspec-crispy in this pull request (and of course in this repository). Instead, I just fixed these problems I found while writing rspec-crispy.

• https://github.com/igrep/crispy/commit/3a1ce2dfc25ba48d07505b6ed1d7125f393e9579 : fix bug which causes a crash when some of @spies_to_reset gets GCed just before reinitialize.
• https://github.com/igrep/crispy/commit/4719652d35292749b76fd2531bfec83a4d383a66 : raise error when given some non spied object (or class) by mistake.

Sorry for too many commits for the size of patch.