Review usage of `ffi::secp256k1_context_no_precomp`

[tcharding]

I'm out of my depth here but after we remove the context from the API should we be using with_global_context instead of ffi::secp256k1_context_no_precomp?

[apoelstra]

Yeah, let's just throw away secp256k1_context_no_precomp. It no longer serves a purpose.

[tcharding]

Mad, lets do it after we get #844 in