Revert "Use ast.literal_eval to implement safeEval"

This reverts commit f5d123be5e342d91c2fa964ddae2844bc633d08e.

Fixes https://github.com/behdad/fonttools/issues/164
See issue for details.
Merge commit '8388a2e37ce349dac6555bb824c82723e3b65fbf'