Revert f508bcf with workaround using explicit instantiation.

(Only tested on OS X 10.8.)

Since the problem is undefined symbol in coremodules.o, we could just
explicitly instantiate LString<unsigned char> there. `vmtest` works fine
after this.

Re-enabled string tests and ByteString.stringSearch.
Merge pull request #84 from kennytm/bytestring-search

Re-enable string tests and ByteString.stringSearch on OS X.