Semirings

[chelini]

Currently,
linnea.mul %a, %b { semirings = "min-plus" } ...

better changing to:

linnea.mul %a, %b { add = "my.min.op", mul = "my.add.op" } ...

my.min.op can be any function like f(a, b) = sin(a)/cos(b).

We also want to have casts like: my.add.op<i8,i8,i16> my.min.op<i8,fp16,fp32>