Add core2 support

[devrandom]

Direct core2 support.

For context, see discussions in #127

Note that the no-std feature enables core2 and alloc, but you can still compile without either std or no-std, to eliminate the alloc dependency.

[devrandom]

Compatible with rust-bitcoin/rust-bitcoin#603

[sgeisler]

Looks pretty good, much simpler than the other one 😅 One question that came to my mind: do we really need alloc for everything we do with core2 or could that even be a separate feature? At least our hash impls should not allocate imo.

[devrandom]

Yes, it's definitely possible to have an alloc feature that is additive to the core2 feature.

[devrandom]

I'll followup on the alloc stuff soon

[devrandom]

OK, I added the alloc feature

[TheBlueMatt]

Do we need this as a feature? Users should be able to specify the core2 feature without this.

[devrandom]

removed, although I'm not sure how to best communicate to the developer what are the main features they should be interested in. I expanded the comment.

[TheBlueMatt]

I'm not sure how to communicate features at all - they aren't really mentioned in anything, and just using Cargo.toml doesn't really solve it. Ideally we'd put it in the top-line lib.rs docs.

[TheBlueMatt]

LGTM, aside from dropping the no-std feature.

[sgeisler]

Looks good. Could use some squashing, at least the little alloc fixup.

[devrandom]

ugh, Rust 1.29

[sgeisler]

I think there was a trick for that, was it importing std as alloc in the default case?

[devrandom]

OK, maybe this will do it

[TheBlueMatt]

LGTM. I think maybe we should start documenting all available features in top lib.rs comments, but that isn't specific to this PR, just something I think all our crates should do.

[devrandom]

squashed

[sgeisler]

ACK 5ab0f4b

[apoelstra]

ack 5ab0f4b