Move Base functions out of `/LinearAlgebra/dense.jl`

[oxinabox]

Most of the functions in
https://github.com/JuliaDiff/ChainRules.jl/blob/master/src/rulesets/LinearAlgebra/dense.jl

right now

don't belong there.
As they are loaded even if LinearAlgebra isn't.
Now right now we don't have a Requires.jl block around that,
so it doesn't matter,
but conceptually we could.

Also it is suprising to have to go look for them there.
I know it has caught people out before.

[oxinabox]

Lets hold off on this til after #91 though
since that will cause a bunch of conflicts otherwise.