Double factorial and primorial

[Torbjorn Granlund]

I implemented a variant where all primes live in the heap, perhaps
exactly what Niels suggested.

It is just proof-of-concept code, and the initial prime table is made
naively using trial division.  After that, there is no division or
multiplication.

This code computes all primes < 10^9 in 2s (on a Phenom II 3.2 GHz).
That is about 40 times faster than the current gmp_nextprime code.

The comparison is quite unfair, since the new code has no "nextprime"
interface.  It also uses 100 times more memory.

95% of the time is spent in heap_remove, unsurprisingly.

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