Add inlinability to ByteBuffer getters. (#1220)

Motivation:

In complex ByteBuffer parsing code a surprising amount of cost comes
from retain/release operations that are emitted by the compiler around
calls to readerIndex/writerIndex, readableBytes/writableBytes, and
ByteBufferSlice._lowerBound. These are all straightforward computed
properties based on usableFromInline operations, and so they can be made
inlinable.

Modifications:

Joannis' sample benchmark runtime is halved by eliminating
retains/releases.

Result:

Better, faster parsing code.

Sources/NIO/ByteBuffer-core.swift

@@ -25,7 +25,7 @@ extension _ByteBufferSlice: Equatable {}
 struct _ByteBufferSlice {
     @usableFromInline var upperBound: ByteBuffer._Index
     @usableFromInline var _begin: _UInt24
-    @usableFromInline var lowerBound: ByteBuffer._Index {
+    @inlinable var lowerBound: ByteBuffer._Index {
         return UInt32(self._begin)
     }
     @inlinable var count: Int {
@@ -412,10 +412,10 @@ public struct ByteBuffer {
 
     /// The number of bytes writable until `ByteBuffer` will need to grow its underlying storage which will likely
     /// trigger a copy of the bytes.
-    public var writableBytes: Int { return Int(_toCapacity(self._slice.count) - self._writerIndex) }
+    @inlinable public var writableBytes: Int { return Int(_toCapacity(self._slice.count) - self._writerIndex) }
 
     /// The number of bytes readable (`readableBytes` = `writerIndex` - `readerIndex`).
-    public var readableBytes: Int { return Int(self._writerIndex - self._readerIndex) }
+    @inlinable public var readableBytes: Int { return Int(self._writerIndex - self._readerIndex) }
 
     /// The current capacity of the storage of this `ByteBuffer`, this is not constant and does _not_ signify the number
     /// of bytes that have been written to this `ByteBuffer`.
@@ -643,12 +643,14 @@ public struct ByteBuffer {
 
     /// The reader index or the number of bytes previously read from this `ByteBuffer`. `readerIndex` is `0` for a
     /// newly allocated `ByteBuffer`.
+    @inlinable
     public var readerIndex: Int {
         return Int(self._readerIndex)
     }
 
     /// The write index or the number of bytes previously written to this `ByteBuffer`. `writerIndex` is `0` for a
     /// newly allocated `ByteBuffer`.
+    @inlinable
     public var writerIndex: Int {
         return Int(self._writerIndex)
     }