Fetch Unit Parameterisation

[dANW34V3R]

Currently the fetch unit is parameterised with minimum and maximum instruction sizes, effecting the behaviour of the last 4 tests. The block size should also be parameterised, either as a new value when instantiating the tests or as a function of the instructions sizes. All other tests should subsequently be rewritten to allow for these arbitrary values.

[dANW34V3R]

The following changes should be made here