Integrate `form`

[burrbull]

What are you think about integrate form functionality in svd2rust (optional)?

When this option is enabled we can do not merge mod TokenStreams in one (and then split with form) and write them in different files directly. I think this can give us significant speed-up in this case.

@therealprof

[therealprof]

Yes, I was thinking about that. I was also looking into integrating with rustfmt directly since the generated code is such a hot mess; quite obviously the code generation/parsing cabilities of syn/quote/proc_macro2 were never meant to generate code stored to disk but only non-persistent code to be compiled on-the-fly.