Implement interop integer conversions to be used with type inference

Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmUtils.kt

@@ -4,4 +4,46 @@ internal fun decodeFromUtf8(bytes: ByteArray) = String(bytes)
 internal fun encodeToUtf8(str: String) = str.toByteArray()
 
 fun bitsToFloat(bits: Int): Float = java.lang.Float.intBitsToFloat(bits)
-fun bitsToDouble(bits: Long): Double = java.lang.Double.longBitsToDouble(bits)
+fun bitsToDouble(bits: Long): Double = java.lang.Double.longBitsToDouble(bits)
+
+// TODO: the functions below should eventually be intrinsified
+
+inline fun <reified R : Number> Number.signExtend(): R {
+    val returnValueClass = R::class.java
+    when (returnValueClass) {
+        java.lang.Byte::class.java -> if (this is Byte) {
+            return this.toByte() as R
+        }
+        java.lang.Short::class.java -> if (this is Byte || this is Short) {
+            return this.toShort() as R
+        }
+        java.lang.Integer::class.java -> if (this is Byte || this is Short || this is Int) {
+            return this.toInt() as R
+        }
+        java.lang.Long::class.java -> if (this is Byte || this is Short || this is Int || this is Long) {
+            return this.toLong() as R
+        }
+    }
+
+    throw Error("unable to sign extend ${this.javaClass.simpleName} \"$this\" to ${returnValueClass.simpleName}")
+}
+
+inline fun <reified R : Number> Number.narrow(): R {
+    val returnValueClass = R::class.java
+    when (returnValueClass) {
+        java.lang.Byte::class.java -> if (this is Byte || this is Short || this is Int || this is Long) {
+            return this.toByte() as R
+        }
+        java.lang.Short::class.java -> if (this is Short || this is Int || this is Long) {
+            return this.toShort() as R
+        }
+        java.lang.Integer::class.java -> if (this is Int || this is Long) {
+            return this.toInt() as R
+        }
+        java.lang.Long::class.java -> if (this is Long) {
+            return this.toLong() as R
+        }
+    }
+
+    throw Error("unable to narrow ${this.javaClass.simpleName} \"$this\" to ${returnValueClass.simpleName}")
+}

Interop/Runtime/src/native/kotlin/kotlinx/cinterop/NativeUtils.kt

@@ -21,4 +21,10 @@ fun encodeToUtf8(str: String): ByteArray {
 external fun bitsToFloat(bits: Int): Float
 
 @Intrinsic
-external fun bitsToDouble(bits: Long): Double
+external fun bitsToDouble(bits: Long): Double
+
+@Intrinsic
+external fun <R : Number> Number.signExtend(): R
+
+@Intrinsic
+external fun <R : Number> Number.narrow(): R