Fix usage of BigInteger in DiscordBitSet.value (#864)

DiscordBitSet can only represent positive numbers, so the BigInteger
used for getting a decimal representation can be constructed using the
sign-magnitude representation instead of the two's-complement binary
representation.

This fixes two bugs:

 * DiscordBitSets with an empty data array couldn't be converted to a
   BigInteger because the constructor taking the two's-complement binary
   representation throws a NumberFormatException if the given array is
   empty.
   This bug was reported by @Tmpod:
   https://discord.com/channels/556525343595298817/1147254164469063773

 * DiscordBitSets with a negative Long at the last position in their
   data array would be misinterpreted as negative numbers in the
   conversion to BigIntegers because the two's-complement binary
   representation was used.
   Example where this bug could be observed:
   // printed -9223372036854775808, should be 9223372036854775808
   println(DiscordBitSet(1L shl 63).value)

common/src/commonTest/kotlin/BitSetTests.kt

@@ -1,6 +1,8 @@
 package dev.kord.common
 
 import kotlin.js.JsName
+import kotlin.random.Random
+import kotlin.random.nextLong
 import kotlin.test.*
 
 class BitSetTests {
@@ -99,4 +101,29 @@ class BitSetTests {
             DiscordBitSet(0b1011, 0b111001, 0b110).binary,
         )
     }
+
+    @Test
+    fun value_works_for_DiscordBitSet_with_empty_data_array() {
+        val bits = DiscordBitSet(data = LongArray(size = 0))
+        assertEquals("0", bits.value)
+    }
+
+    @Test
+    fun value_works_for_all_single_bit_Longs() {
+        for (shift in 0..<Long.SIZE_BITS) {
+            val value = 1L shl shift
+            val bits = DiscordBitSet(value)
+            assertEquals(value.toULong().toString(), bits.value)
+        }
+    }
+
+    @Test
+    fun value_is_never_negative() {
+        for (size in 1..10) {
+            val data = LongArray(size)
+            data[size - 1] = Random.nextLong(Long.MIN_VALUE..-1)
+            val bits = DiscordBitSet(data)
+            assertTrue(bits.value.all { it in '0'..'9' })
+        }
+    }
 }

common/src/jvmMain/kotlin/DiscordBitSetJvm.kt

@@ -7,7 +7,7 @@ internal actual fun formatIntegerFromLittleEndianLongArray(data: LongArray): Str
     // need to convert from little-endian data to big-endian expected by BigInteger
     val buffer = ByteBuffer.allocate(data.size * Long.SIZE_BYTES)
     buffer.asLongBuffer().put(data.reversedArray())
-    return BigInteger(buffer.array()).toString()
+    return BigInteger(/* signum = */ 1, /* magnitude = */ buffer.array()).toString()
 }
 
 internal actual fun parseIntegerToBigEndianByteArray(value: String): ByteArray = BigInteger(value).toByteArray()