Adds Int(buffer:) initializer to FixedWidthInteger types (#3047)

### Motivation:

Makes it possible to make integers out of ByteBuffers directly with an
initializer. I.e. `UInt32(buffer: ByteBuffer(bytes: [0, 1, 2, 3]))`.
Closes #3012.

### Modifications:

- Adds the `Int(buffer:)` initializer in `ByteBuffer-int.swift`
- Adds tests in `ByteBufferTest.swift`. _Holy hell this file is huge._
- Note that `Int(buffer:)` will crash if the buffer does not have enough
bytes in it to represent the desired integer type.
- It also does _not_ expose endianness and uses `Endianness.host`, to
keep the public API clean and simple. It's a convenience initializer, so
I thought keeping it minimal is a good idea.

### Result:

Nicer direct initializer for integers from ByteBuffers.

---------

Co-authored-by: Johannes Weiss <[email protected]>
Co-authored-by: Cory Benfield <[email protected]>

Sources/NIOCore/ByteBuffer-int.swift

@@ -116,6 +116,7 @@ extension ByteBuffer {
 }
 
 extension FixedWidthInteger {
+
     /// Returns the next power of two.
     @inlinable
     func nextPowerOf2() -> Self {
@@ -134,6 +135,21 @@ extension FixedWidthInteger {
 
         return 1 << ((Self.bitWidth - 1) - self.leadingZeroBitCount)
     }
+
+    /// Initialize an integer from a byte buffer of exactly the right size.
+    /// The bytes will be read using the host system's endianness.
+    ///
+    /// - Parameters:
+    ///   - buffer: The ByteBuffer to read from
+    ///   - endianness: The endianness to use when reading the integer, defaults to the host system's endianness.
+    @inlinable
+    public init?(buffer: ByteBuffer, endianness: Endianness = .host) {
+        var buffer = buffer
+        guard let value = buffer.readInteger(endianness: endianness, as: Self.self), buffer.readableBytes == 0 else {
+            return nil
+        }
+        self = value
+    }
 }
 
 extension UInt32 {

Tests/NIOCoreTests/ByteBufferTest.swift

@@ -3499,6 +3499,29 @@ extension ByteBufferTest {
 
 }
 
+// MARK: - Int / FixedWidthInteger init
+extension ByteBufferTest {
+    func testCreateIntegersFromByteBuffer() {
+        let uint32BufferLE = ByteBuffer(bytes: [0x04, 0x03, 0x02, 0x01])
+        let uint32BufferBE = ByteBuffer(bytes: [0x01, 0x02, 0x03, 0x04])
+        let tooSmallInt32Buffer = ByteBuffer(bytes: [0x01, 0x02, 0x03])
+
+        XCTAssertEqual(Int32(buffer: uint32BufferLE, endianness: .little), 0x0102_0304)
+        XCTAssertEqual(Int32(buffer: uint32BufferBE, endianness: .big), 0x0102_0304)
+        XCTAssertNil(Int32(buffer: tooSmallInt32Buffer))
+
+        let uint64BufferLE = ByteBuffer(bytes: [0x08, 0x07, 0x06, 0x05, 0x04, 0x03, 0x02, 0x01])
+        let uint64BufferBE = ByteBuffer(bytes: [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08])
+        let tooSmallInt64Buffer = ByteBuffer(bytes: [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07])
+        let tooBigInt64Buffer = ByteBuffer(bytes: [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09])
+
+        XCTAssertEqual(Int64(buffer: uint64BufferLE, endianness: .little), 0x0102_0304_0506_0708)
+        XCTAssertEqual(Int64(buffer: uint64BufferBE, endianness: .big), 0x0102_0304_0506_0708)
+        XCTAssertNil(Int64(buffer: tooSmallInt64Buffer))
+        XCTAssertNil(Int64(buffer: tooBigInt64Buffer))
+    }
+}
+
 // MARK: - DispatchData init
 extension ByteBufferTest {