Fix miner constructor params and proving period offset calculation

[austinabell]

Summary of changes
Changes introduced in this pull request:
Bugs:

• Miner constructor params were not being wrapped in init::ExecParams when calling init exec method
• miner constructor was checking empty params (even though params are deserialized)
• assign_proving_period_offset was panicking when writing to the address bytes (because it was attempting to write over the address bytes and would fail if the current length was not enough to write a BE u64 value) so I switched to using the buffer writing variant
• Offset was not read and % from a unsigned integer, so values were off when calculating period offset

Reference issue to close (if applicable)

Closes

Other information and links