refactor: switch reqs::Order to owned String

[rschmukler]

This commit changes the internals of the reqs::Order struct to use an
owned struct rather than a lifetime pointer. The main reason for this
change is to allow implementing Into<Order> for structs that don't
have pointer-based representations of strings for products. For example,
consider the following scenario:

enum Coin {
  BTC
  USD
}

enum Pair {
  quote: Coin,
  base: Coin,
}

enum AppOrder {
  pair: Pair,
  size: f64,
}

If you imagine trying to implement From<AppOrder> for reqs::Order it
is currently impossible, as you would need to render the pair (eg.
format!("{}-{}", pair.quote, pair.base) but the generated string would
not live long enough to satisfy the lifetime requirement of the
Order<'a>.

Since an order is almost always followed by IO, and the frequency of
making orders is relatively sparse, the performance overhead of
potentially copying a string unnecessarily seems like a worthwhile
sacrifice for ergonomics.

I realize that this is a breaking change and that it might not be worth releasing immediately (or at all) so please reject this PR if you don't think it's worth it. That being said, I did run into this issue when using it in my application - and I think any application that tries to write bindings to multiple exchanges (and thus has it's own "OrderRequest" type) will run into something similar to this.

[inv2004]

@rschmukler

I think that I would not like to loose &str, because in my code it's &str from top most of the time, and I would like to avoid cloning for performance reasons.

I decided to use Cow (with Into<Cow<'a, str> in constructor).

Would you like to change the PR? or I can create my own.

[rschmukler]

Wow! Very clever way of getting the best of both worlds. A bit odd since internally it’ll never be written, but I think it solves it well! Thanks for your help on it.