File I/O support

[azonenberg]

We should support both old-school CSV as an interchange format and a protobuf-based (?) native serialization format.

File format design is nontrivial because channels may be sparse, have different sampling rates, etc. We should also save protocol decoder config in some way.

[bvernoux]

Why not using something "standard" like sigrok file format for in/out and decoders ?

• See https://www.sigrok.org/wiki/Input_output_formats
• See https://sigrok.org/wiki/Protocol_decoders