[v3] Redo gamma_distribution, other tweaks

[Paolo Carlini]

Hi,

as promised, this is the first stab at re-implementing
gamma_distribution + other relatively minor fixes. For now the new code
is only lightly tested (inspected some accurate histograms for 10 values
of alpha for new vs old implementation), but I'm working on the
testsuite... Tested x86_64-linux multilib, committed to mainline.

Paolo.

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