Test coverage to verify bytes-like object acceptance for primary classes

[tomato42]

Verify that all methods of SigningKey and VerifyingKey that should accept bytes-like objects actually do accept them.

That means: bytes, str (py2.x only), bytesarray, memoryview (buffer of py2.x) and array.array

note that array.array and memoryview of it can have elements that are bigger than one byte (as in a[0] can return ints > 255), for consistent operation, they need to be cast() to 'B' (unsigned char)

the doc strings will likely need to be updated to warn about endianess when array.array is used with multi-byte objects

See also: https://docs.python.org/3/glossary.html#term-bytes-like-object