SI-7186 Slim down some TypeRefs by 8 bytes.

By virtue of being defined in object TypeRef, they
had two outer pointers.

    $outer  scala.reflect.internal.Types$TypeRef$
    $outer  scala.tools.nsc.Global

This commit moves them out of that object. They also
now are no longer shrouded in anonymity, which is
pleasant when debugging and profiling.

The shallow size leaderboard now looks like:

```
   Class             % Shallow  % Objects
                          Size
-----------------------------------------
1. ::                      14%        25%
2. char[]                  10% a       5%
3. ClassArgsTypeRef         6%         5%
4. Object[]                 6% b       1%
5. TypeHistory              5%         6%
6. AbstractTypeSymbol       4%         2%
7. AbsractNoArgsTypeRef     3%         2%
8. j.l.Class                3%         2%
9. TermSymbol               2%         1%
10. HashEntry               1% c       1%
```
a) 6% of the char[] by size is the name table.
   Retained source code seems to be sizable => should we discard after parsing?
b) Types.uniques
c) ZipArchive#DirEntry.entries

Confirmation of the slimmer size can be seen in:

  https://github.com/retronym/scala/compare/ticket/7186

In that branch, I introduced infrastructure to query JVM
object size in instrumented tests, and the diff shows:

```patch
       size                class instance
 ========================================================================
-  64 bytes  internal.Types$TypeRef$$anon$5 List[Int]
-  64 bytes  internal.Types$TypeRef$$anon$6 String
+  56 bytes  nternal.Types$ClassArgsTypeRef List[Int]
+  56 bytes  ernal.Types$ClassNoArgsTypeRef String
```

I chose not to bring that test infrastructure across to master so as
not to burden us with brittle tests built on esoteric facilities.

This does not close the ticket which proposes a different
space optimization which is also explored in that branch,
but no a clear cut improvement.

Review by @gkossakowski (if you're up to it) or @paulp