Replace HttpTryFrom with std::convert::TryFrom

[seanmonstar]

[lovasoa]

Would it be possible to implement TryFrom for all types that implement HttpTryFrom without breaking backward compatibility, using conditional compilation ?

https://dev.to/jeikabu/rust-tryfrom-and-conditional-compilation-46fe

[seanmonstar]

@lovasoa it's possible! We're just moving towards 0.2 already, and in that branch, we don't need to support the older compiler. See #308 (comment)

[seanmonstar]

Completed in the 0.2.x branch.