Make integer division round down

Some integer division was rounding to zero, other was rounding
down. For proper indexing when doing stencils, you want it to
round down.
Removed debugging code
The ordering of args in debug_to_file was inconsistent
Switched to soft float abi for arm
Break do_task out into its own function

So that it can be overridden by the user
Large overhaul of C backend

Now emits ssa-style C.
Does the correct thing for integer division.
Does a little CSE so the source isn't so huge.
Updated vector math test to test division correctly.
Fixed CodeGen_C test to work with new CodeGen_C
Fixed signed divide by constant to work with new division strategy