BigInt operations: trailing_{zeros,ones} and >>>.

I chose to define >>> as fundamental and then provide a dispatch
for >> in terms of >>> because the implementation of the shift for
BigInts is inherently arithmetic, not logical. There is actually
no logical shift operator for BigInts, I wonder if we should define
it at all.
CPU_CORES: pretend there are any number via JULIA_CPU_CORES env var.
Merge branch 'master' of github.com:JuliaLang/julia