Add bitSizeMaybe, FiniteBits to Data.Bits.

These changes allowed me to install nats-0.2, which had previously
failed with "‘bitSizeMaybe’ is not a (visible) method of class ‘Bits’".
Expose Data.Coerce.

This allowed me to install semigroups-0.15.4.
Merge pull request #266 from joelburget/bits

Add bitSizeMaybe, FiniteBits to Data.Bits.
Merge pull request #267 from joelburget/coerce

Expose Data.Coerce.
Detect and run tests in sandbox if present

Instead of assuming a global install, use the sandbox to run tests when
available. Mimics behaviour of cabal install.

Signed-off-by: Rohan Jain <mail@rohanjain.in>