Add fac! macro for factor construction

[contagon]

Following my new-found love for proc-macros, this adds the fac! macro for easy factor creation. Essentially this,

let factor = FactorBuilder::new2(res, X(0), X(1))
    .noise(noise)
    .robust(robust)
    .build();

Now becomes this,

factor = fac![res, (X(0), X(1)), noise, robust];

but still retains the symbol type checking, residual input # checking, and residual output/noise dimension checking.