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)

parser/src/gen/parse.rs

@@ -1,4 +1,4 @@
-// File automatically generated by ast/asdl_rs.py.
+// This file was originally generated from asdl by a python script, but we now edit it manually
 
 impl Parse for ast::StmtFunctionDef {
     fn lex_starts_at(