Change structural types scheme

[odersky]

Make Selectable a marker trait without any members.

[odersky]

I reverted the previous attempt to use import language.reflectiveCalls as an alternative way to import the reflectiveSelectable conversion. It caused ambiguities. I briefly considered to make
language.reflectiveCalls the only way to produce a reflective.Selectable, but decided against it. Doing this would be a useful hack in the short run, but would destroy all invariants we have for the language` module: Namely, that it is compile-time only and that every entry is mirrored by a command-line option.

[sjrd]

Thank you @odersky. I have some small comments. The implementation looks good, but I really think we should have a test with contextual parameters in the selectDynamic and applyDynamic methods, which was at least half of the point of this new scheme.

[sjrd]

There's a SemanticDB test that failed in bootstrap:

[info] Test dotty.tools.dotc.semanticdb.SemanticdbTests.expectTests started
[error] check file /__w/dotty/dotty/tests/semanticdb/expect/Advanced.expect.scala does not match generated.
If you meant to make a change, replace the expect file by:
  mv /__w/dotty/dotty/tests/semanticdb/expect/Advanced.expect.scala.out /__w/dotty/dotty/tests/semanticdb/expect/Advanced.expect.scala
inspect with:
  diff /__w/dotty/dotty/tests/semanticdb/expect/Advanced.expect.scala /__w/dotty/dotty/tests/semanticdb/expect/Advanced.expect.scala.out
Or else update all expect files with
  sbt 'dotty-compiler-bootstrapped/test:runMain dotty.tools.dotc.semanticdb.updateExpect'
[error] Test dotty.tools.dotc.semanticdb.SemanticdbTests.expectTests failed: java.lang.AssertionError: 1 errors in expect test., took 1.397 sec
[error]     at dotty.tools.dotc.semanticdb.SemanticdbTests.runExpectTest(SemanticdbTests.scala:82)
[error]     at dotty.tools.dotc.semanticdb.SemanticdbTests.expectTests(SemanticdbTests.scala:39)
[error]     ...
[info] Test run finished: 1 failed, 0 ignored, 1 total, 1.398s

but otherwise LGTM :)