remove more int64 usage

[tersec]

#2366

[mratsim]

Should we introduce some saturated math type?

As is, this needs a comment explaining why we complexify things, the bounds are quite tricky here:

• inclusive end, which lead to off by 1
• need to think about unsigned underflow
• need to start at 1 and so need to filter out 0 from the range

[tersec]

8825b9a

There's already toGaugeValue() and clamp(); what would this saturated type add? Most of the remaining items in #2366 need non-saturating arithmetic, to the extent they're issues at all (e.g., I only included the genesis time one for completion).