xdelta3: update to 3.1.0.

First release of the 3.1.x series. This is taken from the "64bithash"
branch.

    Adds support for -B values greater than 2GB, enabled by -DXD3_USE_LARGESIZET=1 variable
    Adds new performance and speed regression test (written in #Golang)

When compiled for large sizes, xdelta3 uses a 64bit checksum
function. This impacts both compression and speed.

Relative to 3.0.11, the new branch is currently 3-5% slower and
has 1-2% worse compression. Performance will be addressed in future
3.1.x releases.

(wiz)