Fix example dependency list

[tcharding]

Example relies on rand-std not plain rand dependency.

I do not understand why the following command passes without this patch
applied

cargo test --no-default-features --features=std,rand,bitcoin_hashes

But if we put the same code in a standalone binary it fails as expected?

Since the running of this test is unusual and it is primarily meant as
an entry point example to the library, remove the mention of "alloc"
feature and just depend upon "std".

Fixes: #395

[apoelstra]

ACK 3c9dd2f

If I understand the issue correctly, this example would not have have worked with alloc anyway, since it depends on std implicitly through rand-std.

Edit: forgot "not"

[tcharding]

Its the "rand" vs "rand-std" thing that this PR really fixes.

I may have confused things by removing "alloc" at the same time, my bad. I dropped the "alloc" because I'm wondering if all the recent feature gates I changed were incorrect, it seems confusing to not have "std" enabled but have "rand-std" enabled, even if technically this all works. Why would anyone ever do this. But changing them all again seems like code churn, not sure what to do?

[apoelstra]

@tcharding lol, I think the code churn would be ok. We haven't actually done a release yet.

My bad as well, for reading your changes and not noticing that rand-std and alloc is a nearly-incoherent combination ... although as you say, it does technically work.

For readers: we are talking about the recent changes in #386 commit 6, which replaces a bunch of rand feature gates with rand-std, but then doesn't change the simultaneous any(alloc, std) feature gate to std. This only affects documentation example code.