Removes carriers.

[jcchavezs]

This PR removes carriers (and interfaces Reader and Writer) as suggested by @lvht.

Reasons are:

1. By providing carriers we also dictate how injectors/extractors should work and for that we add a lot of constraints and complexity around them that they might not need. In addition, there is no specific structure for baggage items so forcing them to use an strict carrier will add some overhead we want to avoid.
2. There is a lot of noise around how PHP frameworks work with headers so the arguments of providing a standard carrier that will work in most of the cases is invalid. For example, Symfony request lowercase the header keys so a custom extractor should be written for B3 extraction.

Ping @felixfbecker @beberlei @lvht @tedsuo @yurishkuro @tedsuo

Closes #37

[taoso]

:+1:

[yurishkuro]

@jcchavezs do you mean that you're using standard types as carriers? E.g. in Python the carrier for the TextMap format is the default dictionary. If so, I think that's the right approach, but it doesn't seem to be mentioned in the documentation for the formats - see Go API that clearly states which carrier is expected with which format.

On a separate point, I would suggest implementing a Mock tracer (#40) so that changes like this can be validated in an actual implementation, rather than just the API.

[tedsuo]

This looks good to me. One question: for http headers, is []string more useful that [][]string? I don't think tracers will inject/extract more than one value per header. But if most http header implementations use [][]string perhaps we should match that? I believe I saw this discussed before, and []string is fine. Just double checking.

[felixfbecker]

Can this be merged?

[taoso]

I wish this PR to be merged as early as possible.

[taoso]

@jcchavezs Would you like make a v0.x.x tag? I need to require a release, even it is unstable.

[felixfbecker]

The next version is 1.0.0-beta.2

[taoso]

ping @jcchavezs

[jcchavezs]

https://github.com/opentracing/opentracing-php/tree/1.0.0-beta2

[jcchavezs]

@tedsuo so most implementations use string[][], problem is that there are different ways to get headers, for example getallheaders will return string[] having duplicated values glued by a comma whereas the PSR7 implementation returns a string[][]. I believe every vendor would setup something that works for them based on their propagation model.