Make the interval arithmetic sound

[Mikolaj]

Replace magic numbers like

horde-ad/src/HordeAd/Core/Ast.hs

Line 149 in 826149b

if minb == -1000000000 && maxb == 1000000000

with proper infinities (e.g., a Maybe?). The number currently used in place of infinity is not large enough given how wide rank one tensors can get and not small enough that arithmetic operations don't risk wrapping around. Hat tip to @tomsmeding for getting offended by how hacky this is currently.