Implement minivi, a one-line text editor: given a string of keystrokes, return the final buffer.

Implement 'minivi', a one-line text editor. Given a string of keystrokes, return the final buffer. There are two modes: in command mode h and l move the cursor left and right, b and w jump to the previous or next start-of-word boundary, i enters insert mode, a enters insert mode after moving one character forward; in insert mode characters are inserted at the cursor, '<' deletes the character before the cursor and '~' leaves insert mode (moving the cursor back onto the last character).