* Use SECP256K1_CONTEXT_NONE when creating signing context, as
SECP256K1_CONTEXT_SIGN is deprecated and unnecessary.
* Use secp256k1_static_context where applicable.
...instead of explicitly calling init::{Set,Unset}Globals.
Cool thing about this is that in both the testing and bitcoin-chainstate
codepaths, we no longer need to explicitly unset globals. The
kernel::Context goes out of scope and the globals are unset
"automatically".
Also construct kernel::Context outside of AppInitSanityChecks()
