SI-6476 Improve error on escapement

Behavior of escape processing under string interpolation can
be confusing.  This commit improves the exception message so
you know at least what sort of escapes are handled.

This came up on SO in the form `s"\d".r`, where it may not
be obvious what is throwing and how to work around it.

```
scala> s"\d".r
scala.StringContext$InvalidEscapeException: invalid escape '\d' not one of [\b, \t, \n, \f, \r, \\, \", \'] at index 0 in "\d". Use \\ for literal \.

scala> s"a\"
scala.StringContext$InvalidEscapeException: invalid escape at terminal index 1 in "a\". Use \\ for literal \.
```

Referencing SI-6476 because that has become the magnet ticket
for "escape processing under string interpolation, huh?"

This doesn't address `$"` and doesn't handle the more interesting
parse error `s"a\"b"`.
SI-6476 Documentation

And adjust the test.
SI-6476 Unitize procedures, readability

Strictly trivial updates for readability.

I used to prefer procedure syntax, but since it was
scheduled for removal, I can't scan a def without an
equals sign.
SI-6476 Unitize ALL the procedures!
SI-4563 friendlier behavior for Ctrl+D in the REPL

Closing the REPL with Ctrl+D does not issue a newline, so the user's
prompt displays on the same line as the `scala>` prompt. This is bad.
We now force a newline before closing the interpreter, and display
`:quit` while we're at it so that people know how to exit the REPL
(since `exit` doesn't exist anymore).

The tricky part was to only add a newline when the console is
interrupted, and *not* when it is closed by a command (like `:quit`),
since commands are processed after their text (including newline) has
been sent to the console.
a few missing deprecations in proxy collections.

SeqProxy was properly deprecated, so were the CollProxyLike classes, and
the ones in collection.immutable, but these four somehow survived
the Big Proxy Deprecation (tm).
test for InnerClass and EnclosingMethod attributes

Some parts of the test assert (current) buggy behavior. This is marked
in the test file with TODO. It will be fixed in later work on the
backend.
Merge pull request #3889 from som-snytt/issue/6476-alt

SI-6476 Improve error on escapement
Merge pull request #3902 from gourlaysama/wip/t4563

SI-4563 friendlier behavior for Ctrl+D in the REPL
Merge pull request #3907 from gourlaysama/wip/proxy

a few missing deprecations in proxy collections.
Merge pull request #3927 from lrytz/innerClassesTest

test for InnerClass and EnclosingMethod attributes