Fix some style problems using hlint
a little refraction.

make the "Control/Concurrent/ConcurrentIO/Class" directory and split the origin file
"Control/Concurrent/ConcurrentIO/Class.hs" into two files:
"Control/.../Class/SVar.hs" and "Control/.../Class/AVar.hs"
keep the origin "Control/.../Class.hs" to re-import the two modules above.