apfloat bias and unbias function should be symetric

Currently we have a variety of function for dealing w/ bias which takes a variety of argument types:
- https://google.github.io/xls/floating_point/#apfloatbias: apply bias to the given unbiased exponent
- takes just the unbiases exponent
- https://google.github.io/xls/floating_point/#apfloatunbiased_exponent: return the unbiased exponent from a given apfloat
- takes a whole apfloat value (even if the rest of the apfloat fields are not used)
- https://google.github.io/xls/floating_point/#apfloatexponent_bias: return the bias for the given apfloat type
- take just an apfloat sentinel for type inference

We should unify the different variants and have symetric pair for function (and methods) that can operate on different type of arguments.