Provide a wrapper for aligning lengths of size_t to 4 byte boundaries.
Use heap ram for coinbase in gen_stratum_work, zeroing it before use.
Cache the header length when generating stratum work to avoid calculating it on every work generation, and to only need one alloc+sprintf, speeding up work generation.
Cache the coinbase length to speed up stratum work generation.
Check for calloc failure for completeness in gen_stratum_work.