Eq and Show instances for SBV

[LeventErkok]

It would be nice to remove Eq and Show instances for SBV.

• Show instance seems to be relatively easy to remove; compilation issues can probably be worked around by using "<symbolic>" in the non-concrete case, which is what happens anyhow.

• Eq instance is tricky. We want Bits (SBV a). But in Haskell, Bits class has Eq as its super-class, which necessitates the Eq (SBV a). This is unfortunate. What's not clear to me is why the Bits class require Eq? This seems to be an oversight, and perhaps might be good to take it with the proposals process to see if it can be removed.

[LeventErkok]

Fixed one half of this, by removing the Show instance. The other half (Eq instance) has a new ticket: #301.

[LeventErkok]

Wohoo! Actually the Show instance is kind of "required" since otherwise we can't just apply symbolic value producing functions to constants and have their values shown. Silly me.. Reverted the changes.