Avoid inv() in / and \

[haampie]

This solves a common issue where taking the derivative of f(x::Float32, y::Int) = x / y with respect to x accidentally promotes the number type from Float32 to Float64.

This issue is rather unexpected and hard to discover in Flux.

Ref:

• https://github.com/FluxML/Flux.jl/issues/815
• https://github.com/FluxML/Flux.jl/issues/1026
• https://github.com/JuliaDiff/DiffRules.jl/issues/26