Purescripto

1. Cache the import of a purescript module, in order to avoid its re-import during
importing it for the first time.

1. `--update` will update currently used mirror

1. Suppressing warning of using `is` for CPython 3.8+.

References:
- https://github.com/purescript-python/purescript-python/issues/9
- https://bugs.python.org/issue34850

2. Workaround metadata errors, PureScript compiler produces code with `lineno=0`:

References:
- https://github.com/purescript-python/purescript-python/issues/8


3. Workaround for compating JavaScript incomplete arguments.

References:
- https://github.com/purescript-python/purescript-python/issues/10