* Migrate the Windows Msys2 build from Appveyor to GitHub Actions
* Fix build with newer versions of Msys2 (fix `vasprintf` conflict)
* Enable SQL tests on the Cygwin build on Appveyor
* Fix an error message about Bison not being available when in fact Flex was not available
* Don't fail fast with Mac and Linux GitHub Actions
Quickstart (requires Clang 6 or later):
$ export LIB_FUZZING_ENGINE=/path/to/fuzzing/library.a
$ ./configure --enable-fuzz-testing
$ make
$ cd src/fuzz
$ make fuzz_mdb
$ ./fuzz_mdb
Also add a new `mdb_open_buffer function` to facilitate in-memory
fuzz-testing. This requires fmemopen, which may not be present on all
systems. The internal API has been reworked to use file streams instead
of file descriptors. This allows reading from memory and reading from
files using a consistent API.
