Remove asdl (#21)

This removes the ASDL code generation in favor of handwriting the AST. 

The motivations for moving away from the ASDL are:

* CPython compatibility is no longer a goal
* The ASDL grammar isn't as expressive as we would like
* The codegen scripts have a high complexity which makes extensions time
consuming
* We don't make heavy use of code generation (compared to e.g.
RustPython that generates Pyo3 bindings, a fold implementation etc).

We may want to revisit a grammar based code generation in the future,
e.g. by using [ungrammar](https://github.com/rust-analyzer/ungrammar)

scripts/cspell.sh

@@ -1,3 +1,2 @@
 #!/bin/bash
 cspell "ast/**/*.rs" "literal/**/*.rs" "core/**/*.rs" "parser/**/*.rs"
-cspell ast/asdl_rs.py

scripts/update_asdl.sh

@@ -1,8 +0,0 @@
-#!/bin/bash
-set -e
-
-cd "$(dirname "$(dirname "$0")")"
-
-# rm ast/src/gen/*.rs
-python ast/asdl_rs.py --ast-dir ast/src/gen/ --parser-dir parser/src/gen/ ast/Python.asdl
-rustfmt ast/src/gen/*.rs parser/src/gen/*.rs