Fixes #9: normalize signature before using it to determine classes for unboxed instances.
Fixes #10: introduce "Java.proxy" variants.
The new functions are "Java.proxy_{loader,system,runtime}", that
respectively use: an explicit class loader, the system class loader,
and the class loader used to load the OCaml-Java runtime.
The original "Java.proxy" is a synonym for "Java.proxy_system".
Fixes #11: two single quote are now used to indicate an inner-class.
Fixes #12: pass "-cp" and "-classpath" options when compiling an interface
with "-java-extensions".
Remove useless whitespace.