Update find_unique_channels to work with control-flow sequences.
Compiler for control flow QGL.

Adds a separate compile_control_flow_sequence method for QGL sequences
containing control flow statements.

Renames LLElement to LLWaveform. Adds label property to LLWaveform for later
address resolution.

Moves sequence "normalization" down into the individual sequence level.
Consequently, the channel set gets passed down from the top level.