0.6.0

Breaking changes since 0.5.0:

• LispError is now just Error and is no longer in the prelude
  module.
• LispResult is now just Result and is no longer in the prelude
  module.
• SExp::make_symbol has been shortened to SExp::sym.

New features since 0.5.0:

• sexp! macro for building SExps like a vector.
• eval! macro for less verbose evaluation.