Report inline and outlined generated code size, num functions compiled in `--ujit-stats`

[maximecb]

This is information that would be very useful to have in the stats that we dump with the --ujit-stats command-line argument.

The generated code size is just the current write position in the code block objects, cb->write_pos and ocb->write_pos. We should probably report a value in kilobytes for readability.

The number of functions compiled would be how many functions hit the threshold and had a ujit entry point installed.