make BB.debugDescription print xxd -r -p compatible format

Motivation:

When debugging, I often use ByteBuffer.debugDescription which dumps
the bytes. Unfortunately, the format wasn't compatible with what
`xxd -r -p` expects. Now you can just copy some output, for example

    echo '04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12' | xxd -r -p

and xxd -r -p will correctly parse the byte values.

Modifications:

made BB.debugDescription prefix byte values under 0x10 with a 0 instead
of a space.

Result:

easier debugging with the standard tools

Sources/NIO/ByteBuffer-core.swift

@@ -280,7 +280,7 @@ public struct ByteBuffer {
             for i in Int(slice.lowerBound) + offset ..< Int(slice.lowerBound) + offset + length {
                 let byte = self.bytes.advanced(by: i).assumingMemoryBound(to: UInt8.self).pointee
                 let hexByte = String(byte, radix: 16)
-                desc += " \(hexByte.count == 1 ? " " : "")\(hexByte)"
+                desc += " \(hexByte.count == 1 ? "0" : "")\(hexByte)"
             }
             desc += " ]"
             return desc

Tests/NIOTests/ByteBufferTest+XCTest.swift

@@ -114,6 +114,7 @@ extension ByteBufferTest {
                 ("testLargeSliceBegin16MBIsOkayAndDoesNotCopy", testLargeSliceBegin16MBIsOkayAndDoesNotCopy),
                 ("testLargeSliceBeginMoreThan16MBIsOkay", testLargeSliceBeginMoreThan16MBIsOkay),
                 ("testDiscardReadBytesOnConsumedBuffer", testDiscardReadBytesOnConsumedBuffer),
+                ("testDumpBytesFormat", testDumpBytesFormat),
            ]
    }
 }

Tests/NIOTests/ByteBufferTest.swift

@@ -1331,6 +1331,24 @@ class ByteBufferTest: XCTestCase {
         // So we should still be able to also read the old data.
         XCTAssertEqual(0xaa, buffer.getInteger(at: 0, as: UInt8.self))
     }
+
+    func testDumpBytesFormat() throws {
+        self.buf.clear()
+        for f in UInt8.min...UInt8.max {
+            self.buf.write(integer: f)
+        }
+        let actual = self.buf._storage.dumpBytes(slice: self.buf._slice, offset: 0, length: self.buf.readableBytes)
+        let expected = "[ " +
+                       "00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f " +
+                       "20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f " +
+                       "40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f " +
+                       "60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f " +
+                       "80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f " +
+                       "a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf " +
+                       "c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df " +
+                       "e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff ]"
+        XCTAssertEqual(expected, actual)
+    }
 }
 
 private enum AllocationExpectationState: Int {