Rethink context objects

[sanket1729]

As of latest secp master, there are a few things that we can take advantage of in the rust ecosystem.

Notes pasted from CHANGELOG.

• Forbade randomizing (copies of) secp256k1_context_static. Randomizing a copy of secp256k1_context_static did not have any effect and did not provide defense-in-depth protection against side-channel attacks. Create a new context if you want to benefit from randomization.
• Deprecated context flags SECP256K1_CONTEXT_VERIFY and SECP256K1_CONTEXT_SIGN. Use SECP256K1_CONTEXT_NONE instead.

Few suggestions to get things started:

• Remove context altogether. Create a context as static Option. We can use crates like ctor or implement our own racy initialization with atomics. Contexts are really annoying as they leak everywhere in function signatures.
• We can also keep two versions for each API. One with context param and one without.
• Remove all randomization from the code and simplify our API? People may strong opinions here.

[apoelstra]

cc #388

[tcharding]

First item from suggestions is WIP in #605