Add kak_cursor_{line,column} env var
Buffer: check that newlines are at the end of lines
LineAndColumn::operator+= is not constexpr
NCursesUI::get_key(): move getch_iterator as a local type
assert: add some debug info in the message
add a SIGSEGV handler
Buffer: small style fixes
add more asserts
add undo unit_test and fix Buffer
Buffer: end is either one past last character, or one past last line
Editor: refactor InsertMode::Replace