Change GMPs default PRNG to Mersenne Twister

Since we don't allow users to seed the RNG, there is no BC issue with changing the generator.
Add gmp_random_bits(bits) and gmp_random_range(min, max)
Add test files
Mersenne Twister was added in GMP 4.2
Make sure min < max
Address issues raised by @nikic

 - Thread safety on rand init function.
 - Ret false on validation failure
 - Add _dep of temp_a to temp_b
 - Special case int sized min values
 - More tests!
Merge branch 'PHP-5.6' of https://github.com/php/php-src into gmp-random
Upated NEWS
We can't eliminate FETCH_CONSTANT opcodes for constants represented by AST.
Merge branch 'PHP-5.6'

* PHP-5.6:
  We can't eliminate FETCH_CONSTANT opcodes for constants represented by AST.

Conflicts:
	ext/opcache/Optimizer/pass1_5.c
Deref right value for compound assign ops

Currently we do not support IS_REFERENCE operands for most of our
*_function's (should we?), so we need to deref here.
Fix array/object cast of refcounted tmp var
More fixes for array/object casts with temporary variables
missing include for strlen proto
fix some signed/unsigned mismatch
fix signed/unsigned mismatch
fix signed/unsigned mismatch warning
more signed/unsigned mismatch fix
fix signed/unsigned mismatch warning
fix several datatype mismatch warnings