This enables full C exprs in test/bench define ranges by simply passing
them on to the C compiler.
So this:
defines.N = 'range(1,20+1)'
Becomes this, in N's define function:
if (i < 0 + ((((20+1)-1-(1))/(1) + 1))) return ((i-(0))*(1) + (1));
Which is a bit of a mess, but generates the correct range at runtime.
This allows for much more flexible exprs in range defines without
needing a full expr parser in Python.
Note though that we need to evaluate the range length at compile time.
This is notably before the test/bench define system is initialized, so
all three range args (start, stop, step) are limited to really only
simple C literals and exprs.
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