Add mtx (mutex) to test infrastructure, in order to avoid bootstrapping
complications that would result from directly using malloc_mutex.
Rename test infrastructure's thread abstraction from je_thread to thd.
Fix some header ordering issues.
Integrate the SIMD-oriented Fast Mersenne Twister (SFMT) 1.3.3 into the
test infrastructure.
The sfmt_t state encapsulation modification comes from Crux
(http://www.canonware.com/Crux/) and enables multiple
concurrent PRNGs.
test/unit/SFMT.c is an adaptation of SFMT's test.c that performs all the
same validation, both for 32- and 64-bit generation.
