Ignore byebug's own files when debugging
Fix problems with screen width in the test suite
Bump minitest version
Indentation
Minor changes
Switch to the debug_inspector API :)

Byebug doesn't keep backtrace information anymore, it just asks the
debug_inspector API when required. Byebug still keeps track of the
stack size itself because that is needed permanently for detection of
stop points.

Code is nicer now, byebug doesn't handle duplicated information and it
is even faster: test suite runs about 10% faster now.