Lift and promote with MutableMatrix not completely implemented

[mahrud]

R = QQ[x,y]
K = frac R
m = matrix {{x}}
n = mutableMatrix m

--This works on matrices as usual
lift(m, R)
R_0 * m

--But fails for mutable matrices
lift(n, R)
R_0 * n

--Workaround
mutableMatrix lift(matrix n, R)
promote(R_0, K) * n