add DS_Store to gitignore
start index/slice tests
use assert methods for all tests
BUG: include full range of low ints
prefer isinstance() to explicit type() check
add __iter__ and getvar methods to Context
apply __iter__ and getvar() methods in FrameVM/Context
add index/slice tests
support Advanced Indexing (add op_BUILD_LIST and test)
w => watcher
add support for advanced indexing with bool mask