Also, a few more validation methods were added.

The result type of the `InstanceOf` instruction is always `boolean`.
It is _not_ the type of the argument.

This commit fixes code emission for the `Neg` instruction to also work
with `byte`, `short` and `char` types.

Argument validation was also added to the constructor, and the `Neg.writeCode()`
method uses `impossibleSwitchCase()` to throw an error.

Use `impossibleSwitchCase()` in `Rel.writeCode()`.

This is mainly for discoverability in an IDE. Java has the `if` keyword,
so presumably users will type `if` and expect to see the whole range
of conditional generating methods. `unless()` doesn't show there, while
`ifNot()` does.

The `ifNotInstanceOf()` method complements `ifInstanceOf()` in case
the caller needs to do something when the given object is _not_
an instance of given type.

Fields don't always have to be created using class types; primitive
types or array types are perfectly legal. So we need to correctly
initialize the `Signature`, which isn't always a `ClassTypeSig`.
Fortunately, `Signature` has the right factory method to create
a signature out of a `ClassDesc`.

For now, we only remember descriptors of added members (instance/static
fields, instance/static methods, constructors) and provide public access
to them.

In the future, we might want to remember all added creators, to be able
to prevent adding the same member multiple times, but even in that case,
we should still only expose access to descriptors.

First off, `arrayHashCode()` and `arrayToString()` methods were added
to complement their `expr*` counterparts.

The `expr*` methods never look for multidimensional arrays; they treat
arrays as other reference types.

The `array*` methods, on the other hand, always look for multidimensional
arrays and use the proper `Arrays.deep*` methods if necessary.

This is an equivalent of `EqualsHashCodeToStringGenerator` from Gizmo 1.x.