Tooling to generate interpreters

What's currently here:

• lexer.py: lexer for C, originally written by Mark Shannon
• plexer.py: OO interface on top of lexer.py; main class: PLexer
• parser.py: Parser for instruction definition DSL; main class Parser
• generate_cases.py: driver script to read Python/bytecodes.c and write Python/generated_cases.c.h

The DSL for the instruction definitions in Python/bytecodes.c is described here. Note that there is some dummy C code at the top and bottom of the file to fool text editors like VS Code into believing this is valid C code.

A bit about the parser

The parser class uses a pretty standard recursive descent scheme, but with unlimited backtracking. The PLexer class tokenizes the entire input before parsing starts. We do not run the C preprocessor. Each parsing method returns either an AST node (a Node instance) or None, or raises SyntaxError (showing the error in the C source).

Most parsing methods are decorated with @contextual, which automatically resets the tokenizer input position when None is returned. Parsing methods may also raise SyntaxError, which is irrecoverable. When a parsing method returns None, it is possible that after backtracking a different parsing method returns a valid AST.

Neither the lexer nor the parsers are complete or fully correct. Most known issues are tersely indicated by # TODO: comments. We plan to fix issues as they become relevant.