ot::Spinel::FrameWritePointer class

Defines a frame write pointer. Defines the minimum set of APIs used by `Encoder/Decoder` for writing an encoded/decoded frame. It is simply a wrapper over a pointer into a buffer indicating where next byte should be written. Along with a write pointer, this class stores a remaining length variable indicating number of remaining bytes that can be written into the buffer. Two template sub-class `FrameBuffer` and `MultiFrameBuffer` are defined which respectively allow storing a single frame or multiple frames (FIFO queue of frame) in a buffer of a given size.

Found 2 other functions taking a FrameWritePointer argument: