Promotes the optional @method to a required method, so User no longer needs method_exists() to call it. BC break: existing IdentityHandler implementations must now declare getGuestIdentity().

Until now __get() returned null for an unknown key and, because it returns
by reference, silently created that key in the data (and thus in the storage).
It now throws MemberAccessException, like any other Nette object.

This also ends the by-ref write pattern $identity->cart[] = $item on a key
that has not been set yet.

Identity class must exist as an alias in order to read it from session

A default is no longer merged into the value, so a tuple can have one.

As with arrays, a key the shape does not describe that holds an array in both
layers is a CannotMerge error instead of a silent guess, unless mergeMode() is
set explicitly. A null layer keeps an array below it and overwrites a scalar,
the same rule Type follows.

Deep merging follows the schema only: ArrayType recurses through its items
schema, and a colliding key holding arrays on both sides with no schema
guidance is a CannotMerge error instead of a silent guess; an explicit
mergeMode() is the declared opt-out. The blind Helpers::merge() no longer
backs Type::mergeValues(); a null layer still loses to an array and beats a
scalar.

min(), max(), pattern(), items() and mergeDefaults() live on NumberType,
StringType and ArrayType; on a plain Type they throw DeprecatedException
saying who has them, closing the deprecations 1.4 opened. Type::complete()
became a template the subclasses hook into (coerce, validate, mergeDefault,
normalizeValue, mergeValues), the items completion moved to
ArrayType::completeItems() and describe() reports the subclass options
tightened by a deprecated range in the expression. TupleType keeps its own
min() and max() and completes its positions in the same hooks.

A union such as 'int|string' or 'scalar' has no options of its own (1.4
deprecated them), so Expect::type() now builds anyOf() of the variants via
the new TypeExpression::format(), and error messages read "int|string" the
way anyOf() reports alternatives. A union of a single kind ('int|float',
'int|null') keeps its subclass and its options, exactly as 1.4 promised.
A directly constructed plain Type still accepts a union and keeps the 1.4
deprecation notice for its options.

Type checks the value against the parsed expression on its own: matches() is
the one place that knows every Kind, and Validators is asked only about named
string formats and the deprecated constructs. Those still work exactly as
before but raise a notice and 2.1 refuses them: a range in the expression
('int:1..5', use min() and max()), a validator name that is not a type
('numeric' and friends, use assert()), and a directly constructed Type for
a kind that has its own class (use Expect::type()). Helpers::validateType()
and the deprecated helpers of Base are gone, getCastStrategy() no longer
consults Validators for the builtin list.