Instead of making it hold the shadow data as another `T`, make it hold
the data as a byte array, and allow it to read the byte array as `T`.
This makes it much easier to make a "read_typed" function in the MMU.
Long doubles are always at least 80 bits wide in memory and it suffices
if we can address these 80 bits, to mark the long double as NAN at the
end of an MMX instruction, so the additional magic using conditional
types is unnecessary.
This implements almost all instructions related to the FPU, including
all MMX instructions as well.
A lot of these were copied and adjusted from the SoftCPU implementation.
The next big milestone would be QNan detection and ShadowValue handling.
