message: Keep the underlying storage capacity when clearing the FlatBufferBuilder. Gives a significant performance boost for serialisation of many small messages.
diff: @@ -127,8 +127,8 @@ public struct FlatBufferBuilder {
  mutating public func clear() {
  _minAlignment = 0
  isNested = false
  - stringOffsetMap = [:]
  - _vtables = []
  + stringOffsetMap.removeAll(keepingCapacity: true)
  + _vtables.removeAll(keepingCapacity: true)
  _vtableStorage.clear()
  _bb.clear()
  }
  