fix optional argument handling
tests for optional arguments
note that environment symbols and optional arguments are now compilable
destructuring bind argument tests
added dcar and dcdr instructions (car and cdr for destructuring binds)
use dcar and dcdr instructions to perform destructuring binds
dcar and dcdr instructions
implement dcar and dcdr instructions
fix handling of unbound arguments
more tests for corner cases in destructuring binds
destructuring binds are now implemented