Move the existing integration tests into a new lit-based test suite

This is more concise than the old test harness, and drops a dependency
on the old 'llgo' driver.
Remove the llgo driver.
Remove the llgo-build and llgo-dist programs.
Remove the old runtime library.
Move IR generator go files to an irgen directory

This makes things a little more organized by not having IR generation code
in the root directory.
Update the readme files
Remove old entries from gitignore
Merge pull request #203 from pcc/libgo-cleanup

Clean up libgo branch in preparation for merge into master