dialyzer: Introduce nominal types

This PR implements [EEP-69: Nominal Types](https://www.erlang.org/eeps/eep-0069)
in Dialyzer. As a side effect, nominal types can encode opaque types. We
changed all opaque-handling logic and improved opaque warnings in
Dialyzer.

All existing Erlang type systems are structural: two types are seen as
equivalent if their structures are the same. Type comparison are based
on the structures of the types, not on how the user explicitly defines
them. For example, in the following example, `meter()` and `foot()` are
equivalent. The two types can be used interchangeably. Neither of them
differ from the basic type integer().

    -type meter() :: integer().
    -type foot() :: integer().

Nominal typing is an alternative type system, where two types are
equivalent if and only if they are declared with the same type name.
The EEP proposes one new syntax `-nominal` for declaring nominal types.
Under nominal typing, `meter()` and `foot()` are no longer compatible.
Whenever a function expects type `meter()`, passing in type `foot()`
would result in a Dialyzer error.

    -nominal meter() :: integer().
    -nominal foot() :: integer().

More nominal type-checking rules can be found in the EEP. It is worth
noting that most work for adding nominal types and type-checking
is in `erl_types.erl`. The rest are changes that removed the previous
opaque type-checking, and added an improved version of it using nominal
type-checking with reworked warnings.

Backwards compatibility for opaque type-checking is **not** preserved
by this PR. Previous opaque warnings can appear with slightly different
wordings. A new kind of opaque warning `opaque_union` is added, together
with a Dialyzer option `no_opaque_union` to turn this kind of warnings
off.

This PR is likely not bug-free, but not having it in master is blocking
other changes to Dialyzer. Once nominal type-checking and the new opaque
type-checking are applied to more codebases, we will fix the bugs as
they appear.

Co-authored-by: John Högberg <[email protected]>

Author: lucioleKi