R1:9326b4d131ce

Fix some range issues and 32-bit issues with avatar generation

Summary:
Ref T12444. A few issues:

- `x % (y - z)` doesn't generate values in the full range: the largest value is never generated. Instead, use `x % (1 + y - z)`.
- `digestToRange(1, count)` never generates 0. After fixing the first bug, it could generate `count`. The range of the arrays is `0..(count-1)`, inclusive. Generate the correct range instead.
- `unpack('L', ...)` can unpack a negative number on a 32-bit system. Use `&…