This commit adds `Insert()` and `Remove()` methods in `FrameBuilder`.
`Insert` can be used to insert new content in the frame at a given
offset moving any previously written content forward. `Remove()` can
be used to remove previously appended content from the frame moving
any content after the removed bytes backward. This commit also adds
`GetRemainingLength()` to get the remaining length (number of bytes
that can be appended) in the frame buffer, and `SetMaxLength()`. Unit
test `test_frame_builder` is also updated to validate all the newly
added methods.
This commit adds a new class `FrameBuilder` which helps with
construction of frames in a given buffer. It provides helper methods
to append different items to the frame, e.g., bytes from a buffer or
`Message`, a `uint8`, `uint16` or `uint32` value assuming big or
little endian encoding, or a given object. It also provides methods
to overwrite previously appended content in the frame at a given
offset. This class is used in `Lowpan` (replacing `BufferWriter`)
which helps simplify the code. This commit also updates `Appender` to
use the `FrameBuilder` and adds a new unit test for `FrameBuidler`.
Jonathan Hui