GH-2647 stack trace assertions skeleton base - #2687

…afely

This updates the signature of three proxy-returning methods for the
assumptions conduit which enables the proxy implementations to
consume array types safely, which is needed for the next commit.

This should not be a breaking API change as the only public signature
that has changed is now less restrictive than it was previously.

Public API changes:

- SoftAssertionsProvider#proxy signature is now less
  restrictive, having changed from
    (Class<SELF> a, Class<ACTUAL> b, ACTUAL c)
  to
    (Class<SELF> a, Class<? extends ACTUAL> b, ACTUAL c).
  This allows passing array types in for parameter `b`
  correctly without unnecesarry casts.

Internal changes:

- Assumptions#asAssumption signature changed from
    (Class<ASSERTION> a, Class<ACTUAL> b, Object c)
  to
    (Class<ASSERTION> a, Class<? extends ACTUAL> b, ACTUAL c).
  This allows converting instances where b is an array
  type correctly. This also makes this class more typesafe, which
  will simplify maintainability in the future. An explicit cast
  is needed to prevent the internal arguments being ambiguous
  in intent within varargs expansion.

- SoftProxies#createSoftAssertionProxy signature changed from
    (Class<SELF> a, Class<ACTUAL> b, ACTUAL c)
  to
    (Class<SELF> a, Class<? extends ACTUAL> b, ACTUAL c).
  This allows passing in an array type for parameter `b`, which
  previously would not compile.
  (e.g. (..., StackTraceElement[].class, ex.getStackTrace());