v0.2.3: More operators!

• Implement binary or operator "|"
• implement unary operators ~, -, + (last one is internally called nop...)
• Add a couple of tests for these

v0.2.0: parallelism and error on unknown identifiers

• compile() returns Err() on unknown identifiers
• ExprEval can now be passed and used from threads, e.g. when using rayon's par_iter()