Move all intrinsic declarations into intrinsic.rs. Document them

[brson]

Our declarations for "rust-intrinsic" functions are currently located wherever they are needed. It would be more useful and probably more maintainable to just declare them all in the injected intrinsic.rs file. Also, they should be documented, because they are currently pretty mysterious.

[eholk]

At one point during the summer, I started documenting these here: https://github.com/mozilla/rust/wiki/Note-rust-intrinsics

I obviously didn't get real far...

[brson]

intrinsic.rs is injected by the compiler, so I don't think that's a great place for these - still not discoverable. I would rather just put them in core::private::intrinsics now.

[graydon]

non-critical for 0.6, de-milestoning

[z0w0]

What remains to be done for this? The file exists, so would all intristic definitions still remaining in other files need to be changed to use private::intristics before this can be closed?

[graydon]

std::unstable::intrinsics has all these now, the move is done. they are mostly documented, just missing docs on some math fns.

[catamorphism]

backwards-compatible looks wrong; re-nominating for milestone 1, well-defined.

[catamorphism]

Just a bug, de-nominating

[brson]

Done.