Unify native and foreign methods handling?

[Kai Großjohann]

After some time to digest Michael's excellent work on the foreign
methods handling, it has occurred to me that it might be good to
generalize it even further.

Right now, tramp-file-name-handler treats foreign methods differently
than native methods: if it's a native method, then the corresponding
file operation gets invoked directly; but if it's a foreign method,
then there is an additional indirection.

I wonder if it may be useful to treat native and foreign handlers in
a uniform way?

Then Tramp would be divided into a dispatch module which takes
filenames and invokes the right handler, and several file handlers
which do the actual operations.
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file-error; Data: (Opening input file no such file or directory ~/.signature)