Refactor IO, add buffered IO

I wasn't happy with existing implementation of the IO, so I decided
to change it - as a result there is no longer BH_IOOpen and BH_IOClose
and many IO operations are now optional (behind BH_IOCtl).

Finnally implemented buffered IO and fixed size memory buffer IO.

CMakeLists.txt

@@ -80,6 +80,8 @@ endif()
 set(BH_SOURCE
     src/Algo.c
     src/Args.c
+    src/Buffer.c
+    src/Bytes.c
     src/Hashmap.c
     src/IO.c
     src/Math/Box2f.c