Comments are now removed correctly.
Fixed typo: `exec_compile` -> `exec/compile`.
Replaced `file-init` with `textfile0`...

... according to the new prototype structure
Added tests for stack depth.
Reformatted the stack comments for stream and tstream
Made the error messages more descriptive.

The word `?stack-size` now checks for stack over- and underflow.
`later` becomes `LATER`, for better visibility.
Replaced the `eos` words with `valid?` words.
Some small reorganisation. Added word `0<>`.