[Swift] Moves capacity outside of Storage (#8650)

- Cleans up capacity usage within the lib and moves it outside of the Storage

-  Use overflow operators

swift/Sources/FlexBuffers/Writer/FlexBuffersWriter.swift

@@ -143,7 +143,7 @@ public struct FlexBuffersWriter {
   {
     let vec = createVector(
       start: start,
-      count: stack.count - start,
+      count: stack.count &- start,
       step: 1,
       typed: typed,
       fixed: fixed,
@@ -211,7 +211,7 @@ public struct FlexBuffersWriter {
       typed: true,
       fixed: false)
     let vec = createVector(
-      start: start + 1,
+      start: start &+ 1,
       count: len,
       step: 2,
       typed: false,
@@ -598,7 +598,7 @@ public struct FlexBuffersWriter {
 
     var sloc: UInt = numericCast(writerIndex)
     key.withCString {
-      _bb.writeBytes($0, len: len + 1)
+      _bb.writeBytes($0, len: len &+ 1)
     }
 
     if flags > .shareKeys {
@@ -705,7 +705,7 @@ public struct FlexBuffersWriter {
       /// If this vector is part of a map, we will pre-fix an offset to the keys
       /// to this vector.
       bitWidth = max(bitWidth, keys!.elementWidth(size: writerIndex, index: 0))
-      prefixElements += 2
+      prefixElements = prefixElements &+ 2
     }
     var vectorType: FlexBufferType = .key