Better fors - Scala3

Starting in Scala 3.8 or under -preview mode using Scala 3.7, the usability of for-comprehensions has improved.

The biggest user facing change is the new ability to start for-comprehensions with aliases. This means that the following previously invalid code is now valid:

scala

for
  as = List(1, 2, 3)
  bs = List(4, 5, 6)
  a <- as
  b <- bs
yield a + b

The desugaring of this code is the same as if the aliases were introduced with val:

scala

val as = List(1, 2, 3)
val bs = List(4, 5, 6)
for
  a <- as
  b <- bs
yield a + b

Additionally, this extension changes the way for-comprehensions are desugared. The desugaring is now done in a more intuitive way and the desugared code can be more efficient, because it avoids some unnecessary method calls. There are two main changes in the desugaring:

Simpler Desugaring for Pure Aliases: When an alias is not followed by a guard, the desugaring is simplified. The last generator and the aliases don't have to be wrapped in a tuple, and instead the aliases are simply introduced as local variables in a block with the next generator. Previous Desugaring:
scala
```
for {
  a <- doSth(arg)
  b = a
} yield a + b
```
Desugars to:
scala
```
doSth(arg).map { a =>
  val b = a
  (a, b)
}.map { case (a, b) =>
  a + b
}
```
New Desugaring:
scala
```
doSth(arg).map { a =>
  val b = a
  a + b
}
```
This change makes the desugaring more intuitive and avoids unnecessary map calls, when an alias is not followed by a guard.
Avoiding Redundant map Calls: When the result of the for-comprehension is the same expression as the last generator pattern, the desugaring avoids an unnecessary map call. But the equality of the last pattern and the result has to be able to be checked syntactically, so it is either a variable or a tuple of variables. There is also a special case for dropping the map, if its body is a constant function, that returns () (Unit constant). Current Desugaring:
scala
```
for {
  a <- List(1, 2, 3)
} yield a
```
Desugars to:
scala
```
List(1, 2, 3).map(a => a)
```
New Desugaring:
scala
```
List(1, 2, 3)
```

For more details on the desugaring scheme see the comment in Desugar.scala#makeFor.