Allow deserializing strings as sequences to not allocate on strings with escape sequences

[oli-obk]

"\"foo\"" needs to be deserialized into a heap allocated string first and then parsed further.

#318 gives one alternative to the allocation by rewriting the original input.

If the input cannot be modified, another alternative would be to allow deserialize_seq to produce a sequence of characters or bytes instead of going straight for the string.

[clarfonthey]

That's actually an interesting idea: you could definitely do zero-copy for sequences by deserializing to an iterator. Would require upstream serde support, though.

[dtolnay]

I don't think this would require any further upstream support, since SeqAccess is already effectively an iterator.

The only change here would be intercepting deserialize_seq and, if the input contains a string, driving the visitor with a sequence of bytes or chars instead of with a string.

I'm not yet sold on this idea because it would be detrimental to error messages in some cases. For example if you are deserializing struct S { clars: Vec<Clar> } and the input is {"clars":"lololol"}, you should currently get an error that a sequence was expected but the input contained a string, whereas with this change you would get an error that a struct Clar was expected and the input contained a char 'l'. We would need to figure out how to mitigate that.