kernel: Introduce initial kernel C header API

As a first step, implement the equivalent of what was implemented in the
now deprecated libbitcoinconsensus header. Also add a test binary to
exercise the header and library.

Unlike the deprecated libbitcoinconsensus the kernel library can now use
the hardware-accelerated sha256 implementations thanks for its
statically-initialzed context. The functions kept around for
backwards-compatibility in the libbitcoinconsensus header are not ported
over. As a new header, it should not be burdened by previous
implementations. Also add a new error code for handling invalid flag
combinations, which would otherwise cause a crash.

The macros used in the new C header were adapted from the libsecp256k1
header.

To make use of the C header from C++ code, a C++ header is also
introduced for wrapping the C header. This makes it safer and easier to
use from C++ code.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

ci/test/00_setup_env_mac_cross.sh

@@ -17,4 +17,4 @@ export XCODE_BUILD_ID=15A240d
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="-DBUILD_GUI=ON -DREDUCE_EXPORTS=ON"
+export BITCOIN_CONFIG="-DBUILD_GUI=ON -DBUILD_KERNEL_LIB=ON -DREDUCE_EXPORTS=ON"