Duration: support negative durations by prefixing a '-' before the P in ISO format#111

apoelstra wants to merge 4 commits intoGothenburgBitFactory:masterGothenburgBitFactory/libshared:masterfrom

apoelstra:push-sunqtrzuuumkapoelstra/gbf-libshared:push-sunqtrzuuumkCopy head branch name to clipboard