Add two assembly programs to understand BPL runtime.
Don't allow 0 or smaller array sizes.
Assign offsets, add printing functions.
Fix typos.
Add function to write basic header.
Implement up to write statements.

Not yet convinced about dict-based solution to issue of string labels.
Add assembly files to .gitignore.
Generate code for arithmetic op nodes.
Line up comments at column 32.
Fix silly overindentation.
Add code for generating new labels.
Generate code for comparison expressions.
Compile generated assembly in test.
Implement if statements. TODO: refactor gen_stmt()
Generate writelns, refactor gen_stmt()
Generate code for while statements
Support function calls/return statements.
Don't assume that compound statments have statement lists.
Local variable offsets are now negative.
Expression statements.