It is often useful to get a slice of some maximum len, also accepting
smaller results if the source is not large enough.
This also makes the function always operate on byte indices. Before,
it was inconsistently skipping ansi codes sometimes.
This also fixes potential oob writes in panels.c
* Disable colors on Windows because terminal looks bad otherwise
* Use environment on Windows to set PATH and make sure unit tests use
the compiled libraries and not the installed ones
* Define ssize_t on Windows as well by using SSIZE_T
Update test_analysis_block.c
Update test_intervaltree.c
Update test_unum.c
