SI-9006 Scaladoc: explicit companion and package links

The existing navigation mechanisms have proved hard to discover for newcomers
to Scaladoc.

This commit adds textual links in the navigation bar to the docs of the
companion (if defined) and to those of the enclosing package.
SI-9008 Fix regression with higher kinded existentials

Allow a naked type constructor in an existential type if
we are directly within a type application.

Recently, 84d4671 changed nested context creation to avoid passing
down the `TypeConstructorAllowed`, which led to missing kind errors
in code like `type T[({type M = List})#M]`.

However, when typechecking `T forSome { quantifiers }`, we create
a nested context to represent the nested scope introduced for the
quantifiers. But we need to propagate the `TypeConstructorAllowed`
bit to the nested context to allow for higher kinded existentials.

The enclosed tests show:
  - pos/t9008  well kinded application of an hk existential
  - neg/t9008  hk existential forbidden outside of type application
  - neg/t9008b kind error reported for hk existential

Regressed in 84d4671.
Fix typo
Merge pull request #4187 from jedesah/patch-1

Fix typo
Merge pull request #4165 from dickwall/ticket/9006

SI-9006 Scaladoc: explicit companion and package links
Merge pull request #4169 from retronym/ticket/9008

SI-9008 Fix regression with higher kinded existentials