Support fuzz testing (#4)

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.

README.md

@@ -23,6 +23,7 @@ Intended focus areas of this fork:
 
 - [ ] Security / stability / fuzz testing
 - [x] Thread safety\*
+- [x] In-memory database API
 - [x] Removing GLib dependency
 - [x] Improved ODBC compliance
 - [x] Continuous integration with Travis and AppVeyor