WLambda VVal API Improvements for Embedding

0.3.4 (2019-11-20)

• Feature: Version of WLambda is printed to stderr in the executable
  and is available with the std:wlambda:version function at runtime.
• Feature: EvalContext::eval_string() implemented.
• Feature: Added set_vval_method! macro for providing an OO API more easily.
• Feature: Expanded the VVal API with quick access methods, for
  example self.v_i(idx) or self.v_ik(key).
• Feature: Added for function, to iterate over lists, maps, bytes and strings.
• Feature: Added VVal::iter() for nicer iteration over VVal lists/maps from Rust.
• Feature: Added std:str:to_char_vec to convert a string to/from a list of it's unicode code points.
• Bugfix: The arguments passed on the stack weren't cleared properly
  after the call. This affected std:to_drop functions, which weren't
  called when expected.
• Bugfix: Calling references (including VVal::DropVVal) now resolves
  to the inner values of them