interfaces: introduce and use a protocol class for the `mpatch` module
See
f2832de2a46c for details when this was done for the `bdiff` module.
Two things worth pointing out-
1) The `cffi` module "inherits" the `pure` implementation of `patchedsize()`
because of its wildcard import.
2) It's odd that the `mpatchError` lives in both `pure` and `cext` modules.
I initially thought to move the exception into the new class, and make the
existing class name an alias to the class in the new location, but the exception
is created in C code by the `cext` module, so that won't work. I don't think a
protocol class is approriate, because there's nothing special about the class to
distinguish from any other `Exception`. Fortunately, nobody is catching this
exception in core, so we can kick the can down the road.
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