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>

.github/ci-test-each-commit-exec.py

@@ -42,6 +42,8 @@ def main():
         "-DBUILD_BENCH=ON",
         "-DBUILD_FUZZ_BINARY=ON",
         "-DWITH_USDT=ON",
+        "-DBUILD_KERNEL_LIB=ON",
+        "-DBUILD_KERNEL_TEST=ON",
         "-DCMAKE_CXX_FLAGS=-Wno-error=unused-member-function",
     ])
     run(["cmake", "--build", "build", "-j", str(num_procs)])