SI-7853 An unsatisfying fix regression in explicit outer

The attempt to placate pos/t4970.scala in 55c6fd4 roused another
dragon.

We've got two levers here: the type of the symbol of the outer
accessor, and the type of its DefDef.

They have been historically out of sync due to the vaguaries of
finalResultType (which is far less vague since 671e6e03c7), but
the finicky operation of ExplicitOuter now has a hard time when
we try to bring them into line.

This stuff is notoriously difficult to understand because the
trees you see from `-Xprint` show a tpt derived from the method
symbol's info, and discards the actual tpt in the tree.

Rather than letting `DefDef(acc)` call `TypeTree(sym)` and use
`sym.tpe_*.finalResultType`, this commit computes the member type
of the accessor from the current class and explicitly uses that as
the return type of the outer accessor def.

We should try to push this a little deeper. I tried to put it into
`def TypeTree`, but that broke, among others,
run/concurrent-stream.scala. Maybe `def DefDef` and `def ValDef`?

But a localised fix is the right start as it addresses the regression
in a minimal fashion to get the IDE building again.
SI-7853 A less ad-hoc place to call memberType

Rather than localizing the fix to the outerAccessor, this
commit pushed the call to `memberType` into *all* usages of
`newValDef` and `newDefDef`.

The TPT of `applyOrElse` in synthetized partial functions must
be set explicitly to pass the pos/t7853-partial-function.scala.
Otherwise, the as-seen-from ends up cloning the type parameter
`B1` of `applyOrElse` as it transforms (questionably)
its bound from `List[Int @unchecked]` to `List[Int]`.

Partial Function synthesis was already a delicate area, and this
makes things more explicit which could be counted as an improvement.
Merge pull request #2955 from retronym/ticket/7853

SI-7853 Regression in explicit outer
Position#show prints the point.

Can't ensure range position points are meaningful when we never
see them. To limit noise, only print the point when it != start.

  [x:y]    // point=x, start=x, end=y
  [p/x:y]  // point=p, start=x, end=y

I'm open to a different syntax.

Also prints NoPosition as [X] rather than [NoPosition] because
noise is for construction workers and attenders of rock concerts.

Some range position and parser tests are included so we can see
the checkfile change when the forthcoming fix happens (either an
error message improvement or a positional one.)
SI-7854, SI-6768 better parsing/positioning in parser

The parser hole I found while working on the generated positions
serves as the umbrella for a host of improvements. Upgraded
positions assigned during some specific challenging situations mostly
involving the creation of synthetic trees, e.g. for comprehensions
and closures. While doing so improved some error messages.

Eliminated some of the most glaring duplication in the parser.
It's written like there is some payoff associated with being
spectacularly imperative. Not so far.