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_win64.sh

@@ -13,5 +13,5 @@ export PACKAGES="g++-mingw-w64-x86-64-posix nsis"
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="-DREDUCE_EXPORTS=ON -DBUILD_GUI_TESTS=OFF \
+export BITCOIN_CONFIG="-DREDUCE_EXPORTS=ON -DBUILD_GUI_TESTS=OFF -DBUILD_KERNEL_LIB=ON -DBUILD_KERNEL_TEST=ON \
 -DCMAKE_CXX_FLAGS='-Wno-error=maybe-uninitialized'"