CR in string continuation escapes

Rust accepts the following as a simple string literal, even though it contains a CR:"a\␊␍b" (where ␊ indicates an LF control character and ␍ indicates a CR control character).

The FLS defines StringContinuation in §2.4.6.1:2 (fls_wawtu6j3fiqn) as follows:

StringContinuation is the character sequence 0x5C 0x0A (reverse solidus, new line).

With that definition, the above example would be rejected.

The Reference models this situation by treating all the characters which will be removed by the string continuation escape as part of the escape.

I think it would make sense to look at this together with describing how literals containing StringContinuation are evaluated (#590).

NB the Reference has a note (in expr.literal.continuation) saying that the behaviour of string continuation escapes may change in a backwards-incompatible manner in future.

Start with FLS §2.4.6.1:2, the Reference's expr.literal.continuation note, and issue #590. Compare the formal StringContinuation definition with the Reference's treatment of removed characters, then clarify the specification consistently with literal evaluation. Done means the CR behavior and any compatibility implications are explicitly described.