Use something less error-prone to generate thunking code

[davidben]

It takes far too much boilerplate (about 100-odd lines per NPAPI function) to generate a single function's worth of thunks. This is also rather error-prone, though thankfully I haven't found any bugs short of wrong debug messages. It is probably worth generating these with C++ templates or something.

Maybe reuse Chromium's IPC system? Mozilla is already also using it.