Use SPDX license identifiers for license

[leoheitmannruiz]

I'd love to see #221 implemented. This is my go at it!

[leoheitmannruiz]

Oh, and, out of curiosity: in what context is license_url used?

[leoheitmannruiz]

I suppose, being super proper would entail accommodating SPDX license expressions.

crystal, for instance, is licensed Apache-2.0 WITH Swift-exception.

[leoheitmannruiz]

Thanks for the feedback!

What do you think about allowing for SPDX license expressions?

Edit: It seems to me that without them, it may not be possible to describe the licensing of a shard accurately. Such as in the case of crystal.

[straight-shoota]

Expressions sounds good 👍
We don't need to do any specific processing for this, though. This is primarily an informative field anyway.

[leoheitmannruiz]

I'm really sorry for not getting back to you!

How's this?

What about these, now:

https://github.com/crystal-lang/shards/blob/master/src/spec.cr#L216-L224
https://github.com/crystal-lang/shards/blob/master/spec/unit/spec_spec.cr#L29-L37