Borrow field identifiers from deserializer, instead of input

[dtolnay]

Fixes #167.

Bitvec's handwritten Deserialize impls tried to borrow a bunch of strings directly from the deserializer's input, including the field names "order", "head", "bits", "data", "width", "index" and the value of the "order" field for comparing against type_name::<O>().

This sometimes works, for example when deserializing from a JSON &str. However, this is not how derived Deserialize impls work and a lot of formats won't be able to hand out borrowed data from the input like that. For example, when deserializing any data format from a R: std::io::Read. (You can tell because https://doc.rust-lang.org/std/io/trait.Read.html literally has no methods for borrowing data. Everything that this trait does is defined in terms of copying data into a caller-provided buffer, not borrowing it from the underlying i/o object.)

This PR changes the Deserialize impls to borrow from the Deserializer instead, as opposed to the input data. This works efficiently in all formats. Deserializers that operate on i/o resources will let you borrow from the same buffer that they use for obtaining data from the underlying Read object. Deserializers that wrap an in-memory data structure like serde_json::Value will borrow from there without copying or allocation.

[myrrlyn]

This is wonderful, thank you!!