Replaced secp256k1 with coincurve

[hrishikeshio]

Solves #513
Special thanks to @ofek for writing coincurve and for help.

[ofek]

You can completely remove anything to do with contexts. As coincurve uses a global context by default, you don't need to create one yourself unless you're doing multiprocessing. Also note that all contexts are thread-safe.

[hrishikeshio]

@ coredevs please advice if I should remove context or not. The tests run in parallel, so they wont be using same context unless explicitly specified, right?
I personally prefer explicit approach.
@czepluch

[czepluch]

Didn't it work fine with GLOBAL_CTX before? Then I think we should just stick with what we did before.

[ofek]

@czepluch The thing is, coincurve already does that. You don't need to use a context yourself.

[LefterisJP]

@hrishikeshio Can you remove the global context and rebase so that we can merge the PR?

[hrishikeshio]

@LefterisJP Ok will do.

[hrishikeshio]

@LefterisJP rebased. Ready to merge.

[ofek]

@hrishikeshio Another rebase is required unfortunately. Also, please change minimum version to 4.5.1 to include FreeBSD support and ofek/coincurve@6da9164. Just new features.

@LefterisJP @czepluch Are you prepared to merge after final rebase?

Also fyi ethereum/pyethereum#713

[LefterisJP]

@ofek Thank you for the heads up regarding 4.5.1, rebased and added the version change.

[LefterisJP]

Looks good, merging.