Minting function maintainability

[davidad]

• use correct epoch duration (from actors/builtin/miner/policy.go)
• accept fixed-point input (which @zixuanzh can now use to pass fractional network times)
• remove the dependence of tests on network parameters, by accepting {LambdaNum,LambdaDen} as parameters
• introduce a wrapper function for non-test usage that supplies the hard-coded Lambda constants (and also conveniently performs Mul and Rsh in the right order)