Closes GH-79.
Closes GH-80.

Closes GH-85.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

unified is typically, but not always, used to *serialize* when it
compiles.  Compilers don’t always return such a serialized `string` or
`Buffer` though.  The previous behavior was to place the result of
the stringify phase when processing on `file.contents`.  This doesn’t
make sense for non-text.  This change places non-text results on
`file.result`.

Closes vfile/vfile#45.
Closes GH-87.

Closes GH-90.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

* Remove unused type imports

* Restore unused type

This was autofixed by VS Code. The removed type is unused, but it pads another
type.

* Add FrozenProcessor interface

A frozen processor has the same interface as a regular processor, except `use()`
is always disallowed.

* Fix typo