yesod::pi_word: 2 methods to compute 32b words comprising pi

Fabrice Bellard's method in this implementation is almost 50% faster, but
requires extended double precision (non problem on x86_64, may not work
elsewhere). Bailey-Borwein-Plouffe method is slower, but expected to work
with "normal" ieee754 double.

Signed-off-by: Alex Dubov <oakad@yahoo.com>