In this commit phantom types are defined and implemented.
Only a minimal version of phantom term erasure is included.

Phantom types are in latices outside of the normal `Any`/`Nothing` type lattice.
A new phantom lattice can be defined by extending an `object` with `scala.Phantom`.
This trait defines synthetic members
* `protected final trait Any` where this `Any` does not extends `scala.Any`
* `protected final abstract class Nothing extends this.Any`
* `protected final def assume[P >: this.Nothing <: this.Any]: P`

A phantom lattice object can expose any of the phantom members (`Any`, `Nothing`, `assume`)
using an alias but it is not required.

Restriction on lattices:
* Types from different lattice can not be mixed using `&` or `|` types.
* Type parameters must be bounded by types in a single lattice.

Phantom type erasure (minimalistic):
* Type references `Phantom.Any` and `Phantom.Nothing` are erased to `dotty.runtime.ErasedPhantom`
* `Phantom.assume[P]` is erased to `null.asInstanceOf[P]`
* `Phantom` is erased to `dotty.runtime.ErasedPhantomLattice`, removing the members of `Phantom`

This commit removes arguments and parameter of phantom type from all applications.
Arguments are evaluated before the application in case they have side effects.

```
def foo(i: Int, p: SomePhantom) = ???

foo(42, { println("hello"); getSomePhantom })
```

becomes

```
def foo(i: Int) = ???

val x$0 = 42
val x$1 = { println("hello"); getSomePhantom }
foo(x$0)
```

Note that now `def foo(i: Int)` and def `foo(i: Int, p: SomePhantom)` erase to the same signature.

Tests for this commit where added in `Extra tests for Phantoms 1`, they where back ported as the semantics and runtime is not affected.

Erase all `val` and `var` of phantom type. `lazy val` are not transformed as their expansion is done in the backend.

Erasure:
* Assignments are replaced by their RHS
* Local `val` and `var` definitions are replaced by their RHS
* Fields are removed
* Getters return `null.asInstanceOf[dotty.runtime.ErasedPhantom]`

Tests for this commit where added in `Extra tests for Phantoms 2`, they where back ported as the semantics and runtime is not affected.