[RFA 0/5] improve printing of 128 bit ints

[Tom Tromey]

I wanted to improve 128-bit integer support, primarily for Rust,
though I see in Bugzilla that I reported this bug at least twice for C
as well.

Full 128 bit integer support has two main aspects: printing and
arithmetic (including parser support).  This series cleans up
printing, (bug 16225); leaving arithmetic for a future bug (bug
20991).

Previous printing was implemented, but always zero-padded and did not
correctly handle signed values.  This series fixes that and removes
some redundant code as well.

Regtested on the buildbot.

Tom