adding exhaustive random check (quickcheck)
avoid package reload warning (sbcl)
added test `:insert' etc.

+ added test `:insert'
+ use *howmany*
+ use default failure message (`is')
+ removed gen-trisected-rectangle
changed the randomness (avoid error?)

it's not good to check the float value around the threashold
finally, removed tests on r*
Merge pull request #6 from guicho271828/quickcheck

Quickcheck