* passing a non-string secret or non-string hash to any
  CryptContext or handler method will now reliably result
  in a TypeError.

  previously, passing hash=None to many handler identify() and verify()
  methods would return False, while others would raise a TypeError.
  other handler methods would alternately throw ValueError or TypeError
  when passed a value that wasn't unicode or bytes.
  the various CryptContext methods also behaved inconsistently,
  depending on the behavior of the underlying handler.

  all of these behaviors are gone, they should all raise the same TypeError.

* redid many of the from_string() methods to verify the hash type.

* moved secret type & size validation to GenericHandler's encrypt/genhash/verify methods.
  this cheaply made the secret validation global to all hashes, and lets
  _calc_digest() implementations trust that the secret is valid.

* updated the CryptContext and handler unittests to verify the above behavior is adhered to.

was using default policy to store recommended defaults for hashes,
but it only affects CryptContext objects, and users seem to frequent
using the handler objects directly - so going to store recommended
defaults in the handler from now on.

strict keyword
--------------
* GenericHandler's "strict" keyword had poorly defined semantics;
  replaced this with "use_defaults" and "relaxed" keywords.

  Most handlers' from_string() method specified strict=True.
  This is now the default behavior, use_defaults=True is enabled
  only for encrypt() and genconfig(). relaxed=True is enabled
  only for specific handlers (and unittests) whose code requires it.

  This *does* break backward compat with passlib 1.5 handlers,
  but this is mostly and internal class.

* missing required settings now throws a TypeError instead of
  a ValueError, to be more in line with std python behavior.

* The norm_xxx functions provided by the GenericHandler mixins
  (e.g. norm_salt) have been renamed to _norm_xxx() to reflect their
  private nature; and converted from class methods to instance
  methods, to simplify their call signature for subclassing.

misc
----
* rewrote GenericHandler unittests to use constructor only,
  instead of poking into norm_salt/norm_rounds internals.

* checksum/salt charset checks speed up using set comparison

* some small cleanups to FHSP implementation