Add a new `pluggy.callers._MultiCall` implementation which removes
support the implicit `__multicall__` special argument and drops the
recursion required to handle hook wrappers. Rename the original
implementation `_LegacyMultiCall` and load it only when the
`__multicall__` argument is detected in a hookimpl function
signature. Add a deprecation warning whenever the legacy fallback
occurs.

Resolves pytest-dev#23

`_CallOutcome` limits usage due the constructor calling a function
input. Instead add a `classmethod` constructor `_Result.from_call()`
which can be used to get the same behaviour and avoids duplicate types.