add guards for bit vector widths during concrete evaluation

Tested on my 64-bit machine; will run tests on Jenkins slaves
add tests for #73
Partially fixes #73

Due to the limitations of the GHC runtime, we can't get around the
possibility of out-of-memory errors, but we can prevent individual
bitvectors from being too large for the libgmp-backed bignums.

There is now an architecture-dependent check whenever creating a new
`BV` value in the concrete evaluator to ensure the width does not
exceed the GMP limits. If a width is too large, the evaluation returns
to the REPL much like diving by zero.