Stop using StidedMatrix?

[oxinabox]

StridedMatrix is a annoying impossible extend union type.
it is not too hard for a custom array type to implement all the required methods.
(especially if it is a wrapper type that just delegates them all),

However it is not possible for a new type to join a Union.

I suggest that all dispatches on StridedMatrix in the package be replaced with AbstractMatrix.
If that throws an error deeper in the code because they hit some method that actually needs a StridedMatrix and they don't have the methods needed then so be it.

[btmit]

A specific example that would benefit from this change:

n = 8
a = rand(n, n)
a = PDMat(a * a')

b = UpperTriangular(rand(n, n))
X_A_Xt(a, b)

should be faster than

X_A_Xt(a, Matrix(b))

[devmotion]

Was fixed by #163.