Add @check_allocs macro to enforce allocation checks

[topolarity]

This allows a user to enforce that a function will not be called with any set of arguments that would trigger unbounded allocation in its body:

@check_allocs energy(x) = sum(abs2.(x))

@check_allocs function pythag(a, b, c)
    return a^2 + b^2 + c^2
end

const inplace_sum = @check_allocs (x, y) -> x .= x .+ y

TODO:

• Add allocation check to the pipeline
• Add tests for all of the different function syntaxes

Resolves #30