Allow druid_derive macros in druid crate

[emigr2k1]

This makes use of feature extern_crate_self to refer to the current
crate with another name, in this case, druid. This allows druid_derive
macros to expand to something like druid::Data instead of crate::Data.

Closes #650

[emigr2k1]

If putting druid in the namespace is not a good solution, I could implement something like the solution provided in this issue[1].

[1] rust-lang/rust#54647 (comment)

[cmyr]

I haven't seen this solution before, can you link to any information on it? Are there any major drawbacks?

[emigr2k1]

This feature was primarily implemented to solve problems like this. Here[1] is the tracking issue for the feature and here is some discussion[2]. The only drawback is that you have druid in the global namespace (not exported).
Some other crates do this too: yew[3], RustPython[4]
[1] rust-lang/rust#56409
[2] rust-lang/rust#54647
[3] https://github.com/yewstack/yew/blob/c33fe73f6caad8701a40cd488a00ea9e373930d5/src/lib.rs#L82
[4] https://github.com/RustPython/RustPython/blob/098532b283a98e8b98b70fafbed4ec4187311e97/vm/src/lib.rs#L35

[cmyr]

Okay, then I have one additional request for this PR; in druid/src/widget/flex.rs we currently manually implement Data for two types; can you change those to use derive?

[emigr2k1]

I didn't know what to do with this comment

[cmyr]

you can just delete that.

[cmyr]

great, thanks!