feat: support safer size and rate/lockup calculations

[SgtPooki]

synapse should export normalized & safe calculation functions so future devs/tools building on synapse don't run into issues like the one solved by filecoin-project/filecoin-pin#36

In other words:

1. Prevent small value precision loss during conversions to (standard?) TiB unit
2. Prevent large value overflow
3. Provide a standard unit of reference?
4. Apply min/max boundaries at the SDK level (SDK is closest to contracts and should allow consumers to do the right thing easily)

Related #110

[rvagg]

Added as child of #360