Add general case multiplication that uses Toom-Cook

drivers/c.drv

examples/multiprecision/Makefile

examples/multiprecision/add/why3shapes.gz

examples/multiprecision/compare/why3shapes.gz

examples/multiprecision/div/why3shapes.gz

examples/multiprecision/lemmas.mlw

examples/multiprecision/lemmas/why3session.xml

examples/multiprecision/lemmas/why3shapes.gz

examples/multiprecision/logical/why3shapes.gz

examples/multiprecision/mul.mlw

examples/multiprecision/mul/why3session.xml

examples/multiprecision/mul/why3shapes.gz

examples/multiprecision/sub/why3shapes.gz

examples/multiprecision/tests.c

examples/multiprecision/toom.mlw

examples/multiprecision/toom/why3session.xml

examples/multiprecision/toom/why3shapes.gz

examples/multiprecision/util/why3shapes.gz

stdlib/mach/c.mlw