§ General enough special cases

• Also, I feel thanks to thinking about combinatorial objects for a while I've gained some kind of "confidence", where I check a special case which I am confident generalizes well.

void editor_state_backspace_char(EditorState& s) {
    assert(s.loc.line <= s.contents.size());
    if (s.loc.line == s.contents.size()) { return; }
    std::string& curline = s.contents[s.loc.line];
    assert(s.loc.col <= curline.size());
    if (s.loc.col == 0) { return; }
    // think about what happens with [s.loc.col=1]. Rest will work.
    std::string tafter(curline.begin() + s.loc.col, curline.end());
    curline.resize(s.loc.col - 1); // need to remove col[0], so resize to length 0.
    curline += tafter;
    s.loc.col--;
}