Use makeZeroMatrix, not copy constructor, for mutable submatrices

[d-torrance]

This way, we end up calling the default constructor, which creates a new hash code. Previously, we used the same hash code as the original matrix.

Also add a test to ensure that the hash changes.

Closes: #997

Before

i1 : A = mutableMatrix {{10}}

o1 = | 10 |

o1 : MutableMatrix

i2 : B = submatrix(A, {0}, {0})

o2 = | 10 |

o2 : MutableMatrix

i3 : (hash A, hash B)

o3 = (1845190296659672920, 1845190296659672920)

o3 : Sequence

After

i1 : A = mutableMatrix {{10}}

o1 = | 10 |

o1 : MutableMatrix

i2 : B = submatrix(A, {0}, {0})

o2 = | 10 |

o2 : MutableMatrix

i3 : (hash A, hash B)

o3 = (2554347540320848225, 2672540414264377250)