Fix compatibility with java 9+ (#43)

Java 9 introduced something called "covariant return types". This gets
used in java.nio.ByteBuffer: overriden methods from java.nio.Buffer now
return ByteBuffer instead of more general Buffer. Unfortunately this
breaks bytecode generation for the memory: ByteBuffer::limit cannot be
casted to ByteBuffer.(Int) -> Buffer. It can be casted to
ByteBuffer.(Int) -> ByteBuffer, but not in java 8.

The fix is easy enough: upcast the object to Buffer before calling the
method. In bytecode this corresponds to using Buffer::limit method instead
of ByteBuffer::limit. This works both with older and newer java.

The same applies to Buffer::position

compiler/src/main/kotlin/asmble/compile/jvm/ByteBufferMem.kt

@@ -24,7 +24,7 @@ open class ByteBufferMem(val direct: Boolean = true) : Mem {
     override fun init(func: Func, initial: Int) = func.popExpecting(memType).addInsns(
         // Set the limit to initial
         (initial * Mem.PAGE_SIZE).const,
-        forceFnType<ByteBuffer.(Int) -> Buffer>(ByteBuffer::limit).invokeVirtual(),
+        forceFnType<ByteBuffer.(Int) -> Buffer>(Buffer::limit).invokeVirtual(),
         TypeInsnNode(Opcodes.CHECKCAST, ByteBuffer::class.ref.asmName),
         // Set it to use little endian
         ByteOrder::LITTLE_ENDIAN.getStatic(),
@@ -45,7 +45,7 @@ open class ByteBufferMem(val direct: Boolean = true) : Mem {
             addInsns(ByteBuffer::duplicate.invokeVirtual()).
             let(buildOffset).popExpecting(Int::class.ref).
             addInsns(
-                forceFnType<ByteBuffer.(Int) -> Buffer>(ByteBuffer::position).invokeVirtual(),
+                forceFnType<ByteBuffer.(Int) -> Buffer>(Buffer::position).invokeVirtual(),
                 TypeInsnNode(Opcodes.CHECKCAST, memType.asmName)
             ).addInsns(
                 // We're going to do this as an LDC string in ISO-8859 and read it back at runtime. However,
@@ -111,7 +111,7 @@ open class ByteBufferMem(val direct: Boolean = true) : Mem {
                 InsnNode(Opcodes.IRETURN),
                 okLim, // [lim, mem, newlimL]
                 InsnNode(Opcodes.L2I), // [lim, mem, newlim]
-                forceFnType<ByteBuffer.(Int) -> Buffer>(ByteBuffer::limit).invokeVirtual(), // [lim, mem]
+                forceFnType<ByteBuffer.(Int) -> Buffer>(Buffer::limit).invokeVirtual(), // [lim, mem]
                 InsnNode(Opcodes.POP), // [lim]
                 Mem.PAGE_SIZE.const, // [lim, pagesize]
                 InsnNode(Opcodes.IDIV), // [limpages]
@@ -261,4 +261,4 @@ open class ByteBufferMem(val direct: Boolean = true) : Mem {
     override val storeLeavesMemOnStack get() = true
 
     companion object : ByteBufferMem()
-}
+}