Wren implementation

Adding a time function

Since Wren doesn't have a time function, we add a System.gettimeofday function which returns a float with the number of seconds since epoch (with fractions of seconds).

This is done by applying the patch in wren-add-gettimeofday.path to Wren's source code before compiling it (see Dockerfile).

Wren interop

See examples in tests/stepA_mal.mal for usage of wren-eval to evaluate Wren expressions inside a Mal program.