4431df18fe script: remove `unsafe` marker from slice-to-script conversions (Andrew Poelstra)
Pull request description:
The length of the slices is not a safety invariant.
Fixes#3531
ACKs for top commit:
tcharding:
ACK 4431df18fe
Tree-SHA512: 85f745058bc5bf68d312f75c53cef570ead986247ca3cb3fc54b5381dd0214e7df55c1fcb99f06e693b0b9478ec6a9785186cac56886f8007ab7b827d903bd01