ICU4X data structs don't actually borrow any data

[sffc]

I discovered that even though we religiously use Cow<'s, str> and similar in our data structs, we are not actually benefiting from zero-copy deserialization, because the default implementation of Deserialize for Cow<'s, str> is to always use Owned data. See a discussion here: serde-rs/serde#1497

We need our structs to support zero-copy deserialization. This requires adding #[serde(borrow)] to opt in where possible, keeping the following issues in mind: serde-rs/serde#2016, serde-rs/serde#723.

Unfortunately, if we make this change, it means that we will no longer be able to read from an std::io::Read when deserializing. We must always load the buffer and then point into the buffer. This might be an okay constraint for ICU4X, but I want to make sure we are explicit about it.

If we want to retain the ability to deserialize into a 'static, we could add functions that flatten the lifetime on the data struct after we deserialize it from a borrowed buffer.

CC @Manishearth

[zbraniecki]

https://docs.serde.rs/serde_json/fn.from_reader.html

Note that counter to intuition, this function is usually slower than reading a file completely into memory and then applying from_str or from_slice on it. See issue #160.

The ToOwned is relatively easy for flat structs like Vec<Cow>> it becomes challenging for nested structs like AST.
In Fluent Rs I worked it around by making it generic over Slice and Slice is implemented for String or Cow:

• https://github.com/projectfluent/fluent-rs/blob/master/fluent-syntax/src/ast/mod.rs#L116
• https://github.com/projectfluent/fluent-rs/blob/master/fluent-syntax/src/parser/slice.rs

[sffc]

2021-04-15: We are okay with the constraint that deserializers operating on std::io::Read will not work. @Manishearth is investigating some of the upstream bugs. I will work on the changes to ensure that we can achieve zero-copy deserialization on all of the existing data structs.

[sffc]

Closing this as a duplicate of #667