Spin-off rustdoc's tuple code to libstd

[kud1ing]

Fixes #1547

[nikomatsakis]

my opinion is that the "tuple" module should be in libcore, as are other modules named after types (admittedly tuple is a family of types, not a signle type)

[kud1ing]

I thought so too, but there is the desire to keep libcore small.

[brson]

I concede, let's put it in core.

[kud1ing]

Sorry for that merge commit. I somehow merged with myself.

[kud1ing]

Thanks.