Audit and improve unseal for performance

[dignifiedquire]

unseal has not gotten as much attention yet, and there is likely more things we can do.

• avoid a second copy: fix: avoid one in memory copy for unsealing #1401
• use mmap, instead of in memory https://github.com/filecoin-project/rust-fil-proofs/blob/master/filecoin-proofs/src/api/mod.rs#L156-L157
• others?

[cryptonemo]

Related work:

#1401
filecoin-project/filecoin-ffi#162