Add README.md for public release. Implement KILL and NAME statements
for file management. Change test programs to use relative paths
instead of /tmp/ and clean up temp files with KILL. Update .gitignore
for test artifacts.
Add CHAIN statement for loading and running chained programs with
optional ALL flag to preserve variables. Extend RUN to accept a
filename string argument. Implement random-access file I/O with
FIELD, LSET, RSET, PUT#, GET# and the MBF conversion functions
CVI/CVS/CVD/MKI$/MKS$/MKD$. Add COMMON statement (parse and skip).
Five new test programs covering all new features (27 total).
