Get io reader/writer classes only when needed

Structure taken from #771

…nuation

Add line continuation for the WASM demo terminal

…o walrus

- check unicode type by unicode_categories
- rm redundant check of empty string

str.isprintable

By the Python docs, the `!` forces a conversion of the argument before
applying the normal formating logic described after the `:` marker.

See: https://docs.python.org/3.4/library/string.html#format-string-syntax

Feature: str.format accepting !r, !s and !a

Make parser and compiler optional features for vm crate.

Add initial unicodedata module.

Do not pollute stack when if-expression condition evaluated to False

Underscore2hyphen

Add module listing to whats_left.sh

Use importlib import in WASM

Add name, path, msg attributes to ImportError

Remove old import

Allow skipping of unknown modules in whats_left.sh

Remove importlib frames from traceback